I'm getting ready to host a church party with 35-40. Like you I'm going with 4 butts, one of which I can keep in reserve. I'll do a brisket, and 10-12 lbs of legs and thighs. I'm also doing 4-5 lbs of baked beans. We have about 6-7 different sides coming. Good luck and hope the weather is good.
WSM/Red Weber Gasser w/side burner/Weber OT Gold/Large BGE/Hasty Bake Gourmet SS/Lang Original 48 mobile/ Smokey Joe/Super Fast Orange Thermapen