This challenge is inspired by and modeled after the original challenge written by tigger89 for TS3 & the more recent converted challenge written for the Sims 4 by Scklsy.
I wanted to better blend the two challenges together to make the rules more clear and easier to follow & also to make a few changes. I will be following these new rules in my own game play and leave it up to you to choose which version of the rules you like best.

The basic concept of this challenge is that you have four sisters, each of whom represent the four elements: Fire, Air, Water, and Earth. These sisters want to rid the world of the current horrible generation and believe by doing so they will be making the world a better place. Slowly, the sisters will begin to replace the world's population with their own perfect offspring in an attempt to restore balance and peace to the realm. In order to do this they will preserve their everlasting youth by drinking cow-plant milk, eating ambrosia, and collecting death flowers to make them immortal so they can protect the world for centuries to come. The challenge is successfully completed when the sisters have have killed 100 men, and raised 100 babies (or more) from each of these men into young adults. Only one baby from each man can count towards your goal of 100 babys. So if you have twins/triplets from one man the extra one/two babies will be 1 bonus point each but can not count towards you're 100 baby goal. This is because the sisters want to replace every sim they kill with a better sim in order to improve the world. Additional deaths such as friends of the sisters, or family members of the 100 men will not count towards your final goal of 100 dead men.

Begin by creating a new game and creating your four sisters based off the four elements. Its suggested that you name them according to their element and dress them in colors matching their element (however this is of course optional). Examples include: Naming the fire sister: Fira, Blaze, Ember: The air sister: Breeze, Aira, Gale; The water sister: Brooke, Marina, Rain; The Earth sister: Terra, Riku, ila and dressing them in the colors red/orange/black for fire, white/yellow/light blue for air, dark blues and purples for water, greens and browns or floral patterns for earth.

(The sisters do not need to look alike, they can be all different races and ethnicity since they are representing different elements of the whole world however how they look is completely up to you. CC is allowed!) The sisters can be human or alien. If you decide to make the sisters alien they should be in there disguises when outside of their home and try their best to keep from glowing and exposing there true selves to any outsiders. All family members including kids should try to keep their alien identity secret. Try not to abuse the alien powers or abilities to gain any advantage in the challenge
The Sisters must be young adults, must all be related as sisters, and must possess the following traits:

Fire - Romantic, Hates Children, Outgoing
Aspiration must be Love (Serial Romantic) or Popularity (Friend of the World).
Water - Perfectionist, Neat, Family-Oriented
Aspiration must be Food (Master Chef).
Air - Genius, Bookworm, Loner
Aspiration must be Creativity (Bestselling Author) or Knowledge (Nerd Brain).
Earth - Active, Self-Assured, Loves Outdoors
Aspiration must be Athletic (Bodybuilder) or Nature (Angling Ace or Freelance Botanist)

Here are the roles of the sisters:
-When leaving the lot ALL family members have to go. This means all four sisters & all children.
Exceptions include when family members are at work or school or when the babies are stuck at home and sent off to daycare when you leave.
-The fire sister: Her job is to search out men, develop relationships with them, and bear their children. She is allowed to marry but its preferred if they elope and not have a big wedding party or get married on the home lot. It is suggested to try and max out the relationship (green) and romance bar (pink) with each man to add an additional challenge and also so the ghosts of the men do not hate the fire sister when they haunt the home lot. Men are not allowed to move in unless necessary to help with feeding them to the cow plant and all funds brought to the household by them must be removed. The fire sister may bring one man home each night and may try for baby once every 24 hours. Additional woohoo(same and opposite sex) may take place as often as necessary. The fire sister hates children so she will only begin teaching her children when they become teenagers. The fire sister will teach the sims music lessons, painting, & charisma. Skills which she is allowed to develop and use are charisma, Comedy, Mischief, mixology, music and art this includes the DJ skill & Dance skill that came with the sims 4 get together. Her main goal is to harvest cows milk essence of life for her and her sisters. When the children age up to young adults the fire sister will finally accept them and love them and will paint a portrait of them to hang up somewhere in the home. She will be the one who sends them on their way when they move out.

Water: She is the caretaker of the young children and of the house. Cooking, cleaning and repair duties fall to her, as well as the care of young children. Her responsibilities for the children include feeding, socializing and changing diapers of babies. She will also babysit and play with the children with their toys. She will help teach the teens to cook and to take care of them selves. Her main goal is to level up in the cooking skill so that she can begin to create ambrosia for herself and her sisters. Her hobbies include swimming and fishing. It is suggested to try to collect every fish in the game as a mini challenge. She will be focusing on the cooking, gourmet cooking, baking, handiness & fishing skills. Since she is water all bathrooms should be on her side of the house and the other sisters will have to visit her section of the house to use them.
Air: She is the keeper of knowledge. Once children become old enough to learn, she is responsible for teaching them. She reads books to children. Tutors them with skills and homework and lectures them. She is allowed to learn any skill but must learn it only by reading books! Only after she has mastered the skill through reading can she start to use that skill. For example: she must reach level 10 of logic from reading the logic books before playing chess or using any logic objects.
Earth: She is the gardener and athletics master. She is responsible for growing food to sell and eat, collecting collectibles to sell as well as training children in athletics and martial arts. It is suggested to try and complete all collections in game as a mini challenge (frogs, fossils, insects) and maybe even create a room in your home to display them all in.
All sisters can learn the yoga and meditation skills to better foster their immortal powers!

Note: If you wish, 2 out of the 4 sisters may hold a day job. Choose carefully which sisters will go to a job. The Water sister will have to take care of kids constantly, so naturally, she isn't a good candidate. If you find yourself not playing with a sister very often, I would pick her. Also beware of the career you choose for her, due to the skills they are allowed to learn. The Air sister may not make a good chef, but she could make a pretty darn good writer!
NOTE - If social skills (Charisma, Comedy, Mischief) are gained through an uncontrollable event such as socializing with other sims, that is okay but they cannot practice their speech at a mirror, etc. The only sister allowed to practice social skills in such ways is the Fire sister.
NOTE 2 - The sisters are allowed to use their skills to earn money for the household (painting, gardening, fishing etc). But remember, only 2 out of the 4 sisters may get a day job!

You are free to decorate the sisters' house as you wish, but make sure every element has its own location (I made 3 separate buildings and a hedge for Earth, but you can use a big house if you wish, it's up to you). You are allowed to cheat for your initial house (buying or building) but you must have around $1,000 simoleans when you're done, and you cannot buy luxuries other than what is listed below (you must buy those with earned simoleans). The sisters ARE allowed to go to each others' quarters, but they cannot sleep there, or take the responsibility of another (example, Fire can't take care of the kids, Air can't cook, etc). This is a rough description of what every sister should have:
Fire:
fireplaces, 1 double bed, bars, barstools, loveseats, chairs, couches, coffee tables, end tables, dressers, closets
Water:
refrigerators, sinks, stoves, trashcans, tables, counters, chairs, single beds for kids, double bed for self, cribs, toys for the kids, toilets,bathtubs & showers, couches, coffee tables, end tables, dressers, closets
Air:
bookshelves (you have to buy books with your own money), tables, chairs, comfy chairs, couches, love seats, coffee tables, end tables, double bed for self, skill learning objects for children and teens
Earth:
double bed for self, garden area, workout equipment, collectibles area

you are allowed to place down as many as these permitted items as you want in addition to any decorations but NO LUXURY ITEMS ALLOWED! This means NO: pools, fire pits, tvs, stereos, computers, skill building items other than the stove. All luxury items must be earned on your own including easels, guitars, computers, dishwashers, microwaves, popcorn makers, icecream makers, coffee machines. etc!! Appliances can be as high quality as you desire so the most expensive stove is not considered a luxury item.You may also place as many lighting, or decoration(plants, clutter, or other objects that have no use other than looking nice and raising environment score) objects as you want

Gaining immortality - This is achieved by getting the cowplant. The sisters will require one cowplant each, and if a cowplant dies, you must immediately replace it as soon as you can. The cowplant is obtained by: fishing Oasis Springs Desert Bloom Park’s watering hole or in the waters of the Forgotten Grotto;
exploring space with a rocket (which the Air sister should make);
or digging for treasures, which can be harvested by grafting a specific plant type’s cuttings with one another (Daisy & Strawberry gives Dragonfruit, after which Snapdragon & Dragonfruit gives a Cowplant Berry)

The easiest way is to fish for it, but you can do whichever you please For an extra challenge, aim for all 3.

Once you have your cowplant, be sure to feed it once every 12 hours so it doesn't eat one of the sisters! (I recommend saving before feeding the cowplant if you see it has a cake tongue). The Fire sister lures in people with her charm to feed the cowplant. Once the cowplant is fed, you can milk it to get the Essence of Life drink, which adds extra days to your sim's lifespan and makes them instantly happy.

UPDATE: This step isn't required, but will make your sisters truly immortal, and is good to have in case one of your sisters do accidentally have an accident. The Earth sister may learn to craft the seed to, and grow, the Death Flower (this guide may help you, scroll down a bit). Every sister should at least have a couple, so if a sister has an accident, another can save her by bringing her the flower.

NOTE - If you're having trouble getting NPCs to be eaten by the cowplant, you can try moving them in and having them attempt to feed the cowplant themselves (but you must immediately deduct any funds they add to the household in a way that you cannot use). Reminder that if they survive the cowplant, it will still produce milk but this does not give you the "Essence of Life" drink. So a sim must die for the sisters to remain immortal.
NOTE 2 - If you're a little slow-paced like me, you can turn aging off until you get the cowplant.

Object of the challenge: To have as many children as possible while ensuring that they never fail a lifestage(I will say what the criteria for that are). (Main Goal = 100 babys from 100 different men. Killing the 100 fathers of those babys)
Scoring: 1 point per child who grows to young adult and moves out. 1 additional point if the fire sister paints a "portrait" of them(not a still life) as a young adult before they move out, and keeps it on display on the lot.
Ways to lose:
If you fail any life stage.
If any of the sisters or the children die.
If any of the sisters age up to Adult from Young Adult.
If any children are taken away by the social worker.

Responsibilities at each life stage:
Pregnant:
No one can leave the lot!
The only responsibility is to keep the fire sister happy and comfortable.

Infant:
Keep the baby fed, clean, and socially happy.

Child:
must be taught all basic skills from the air sister. Must be read to and tutored with skills and homework by the air sister. The air sister can write books about things such as manners to teach the children.
must be played with by the water sister using the dollhouse, playground, etc.
must be athletically trained by earth (can do this in the pool)
must have 50% relationship with the water / fire / air sister before aging up to teen.
The child must attend school and do homework to work towards an A.
They must also skill up and master their child aspiration.
They should do homework or tutor their skills with the air sister.
Skills to concentrate on at this life stage are logic and art.
They must also develop a relationship with the earth sister, though games or talking, even if there's nothing she can teach them yet.

Teen:
By the end of the first week as a teen, they must have a BFF.
They must find a boyfriend/girlfriend of their choice, and go steady with them.
They also must continue to keep up with school and homework.
They begin music / art lessons with the fire sister. The single guitar may only be placed in her quadrant, and is not allowed to be moved. Just pretend that she is teaching the teen – you can have her “watch” The piano, violin, and now DJ table, and dancing skill can also be used.
They should also help the earth sister with her garden, and receive some athletic or martial arts training from her as well.
They can have their own apartment in the basement and live on their own apart from the sisters and children. They should receive cooking lessons from the water sister, and scheduled tutoring from the air sister.
Must pass school with an A.
GOALS: logic 5, art 5, music 5, cooking 3, handiness 3, gardening 3,athletic 3.

young adult
you should have raised the perfect model citizen to release back out into society. Remember your not just having kids aging them up as soon as possible and moving them out as soon as possible. You want to raise quality adults with good traits, and skills that will make the world a better place then the sims that existed before them. (Example: Don Lothario can be a bit of a well....."bad sim" and he may be a victim of the four immortal sisters but his daughter / son will be raised into a perfect model citizen that will replace him out in the world)

Additional rules:
No cheats.
You may not purchase additional items from buy mode.
You must use normal lifespan.
You can pick the traits of the children, but no 2 children may have the same combination of traits.
You may form groups with the family members or sisters only but cannot join other groups or add sims to your groups.