So I want to make these two men and two women enter a group marriage, but all of the polygamy mods I've seen have completely removed jealousy, which is something I don't want. Is there some mod that allows one to turn off jealousy only for specific people, or any other mod that might help me conduct group marriages while still allowing people to freak out after seeing their partner cheating?

For polygamy this Wedding Arch does not remove jealousy.

To adjust jealousy for specific Sims you could use ACR. You can disable the autonomy for the neighborhood if you don't want it. For jealousy it gives you the options to use the mod settings, no jealousy or maxis settings for the whole neighborhood and the option to change the hood settings for specific sims.