It appears that Maxis is prone to surprise us with changes to the game when we get expansion packages. In Nightlife, it seems they added an extra bone assignment that never was used in the original game and the University EP. The location for the 4th bone assignment to be located was always there, but since there were only 3 bone weight slots, it had gone unutilized.
Heads up edit by Tiggerypum:
When editing the Glamour Vest mesh, I found that there were about 50+ bones in the shoulder/arm area that had 4th bone assignments (read on for how to find them) First I fixed the fingers as Wes suggests below. Then I went through and fixed the rest, dropping the lowest bone setting, which was always 10% or under - and redistributing the points evenly between the other bone assignments to get back to 100%. A bit tedious, and I can see no animation difference in the game. In the meantime we've had other folks post meshes with 4th bone assignments and they APPEAR to work in the base game only environment. So fix or not as you wish.
Someone there figured out that you could interpolate a 4th weight by making the weight for the 4th assignment equal to the sum of the other three weights subtracted from 1.0. (or 100% as displayed in the UniMesh plugins, starting with V4.04).
So I refer you to the picture I have attached. It displays a portion of the left hand mesh with what look like two vertices selected on the ring finger, but are really 4 vertices. In NL they are assigned as l_mid0, l_mid1, l_pinky0 and l_pinky1 and weighted 25% each (the right hand is set up in a similar fashion).
In the original game, these 4 vertices were assigned as 50% l_pinky0, 25% l_mid0 and 25% l_mid1.
Anyone wanting to mod a NL mesh and make it look good in a base game would likely want to change these assignments from the new way to the old way. I don't know what the legal status of distributing expansion pack clothing items to people with only base game packages is, but if it is an allowed practice, then making the suggested changes would be a good practice.
Additionally, there are many locations on new NL hair meshes with 4 assignments, but one or more always seem to be one of the five animation targets that have names ending in _hair. Leaving these assigned to a _hair will make an improperly animated hair mesh that will have some strange and unusual defects. For hair meshes, the ideal solution would be to go through all the bone assignments and remove all references to any _hair joints, and distribute the weighting to the other assigned bones.
If you lack the desire to put in that much effort on your project, then you can assign all the bones to the head, at 100%, or look for some posts in the UniMesh V4.04 thread by Dr. Pixel, where he shows some advised bone assignment and weighting practices for longer hairstyles that are well thought out and should improve the hair mesh appearance.
<* Wes *>
To help identifying the vertices with a 4th assignment, if you are using MilkShape and the UniMesh plugins, there is a test version of a plugin to select all vertices with a 4th bone assignment posted in the following message:
The instructions are somewhat sparse, but all you would need to do is import a mesh with 4 assignments and the use the 'Sel All 4Bone' tool from the Vertex menu.