Assuming you are making baked ham and baked chicken (vs. some type of casserole), here is information that you can use to determine quantities needed for the ham, chicken, baked beans, potato salad, macaroni salad, and scalloped potatoes. I'm also providing you with some recipes for items that you can prepare in advance so that you aren't trying to cook all of these items in such a short period of time. I'm also providing you links to other large quantity recipes that you might consider if you want to make something other than what you mentioned.
Chicken : for cut up chicken, you would need about 350 lbs of chicken plus any other ingredients to season it. Here is a good recipe that will serve 250 people and if you double it, you would have more than enough for 400: http://members.tripod.com/~lotsofinfo/H14easybakedchicken250.htm
Ham: 50 lbs of bone-in-ham will feed 100 people so you would need 200 lbs of bone in ham for 400 people (if you are making both chicken and ham, you might only want to make 200 serviings of each type of meat so that would be 100 lbs of ham)
Baked Beans: Since I don't know whether you want to make this from scratch or by using canned beans, I'm providing a link that gives you both options. The quantity is for 100 people so you would have to quadruple the quantity in the recipe: http://www.ellenskitchen.com/bigpots/beans100.html Here's another BBQ beans recipe for 100 http://www.recipegal.com/sidedish/BBQ-Beans-for-100-People.htm
Potato Salad: 35 lbs of potatoes (plus whatever ingredients you want to add to make the salad mixture) are needed for 100 people so you would need 140 lbs of potatoes for 400 people. In this link is a recipe for potato salad for 100 people that you can quadruple to make enough for 400: http://www.ellenskitchen.com/bigpots/plan/quan100.html
Macaroni Salad: 7 to 8 lbs of dry pasta are needed for 100 people so you would need 28 to 32 lbs of dry pasta for 400. In this link is a recipe for macaroni/pasta salad: http://www.ellenskitchen.com/bigpots/plan/quan100.html
Scalloped Potatoes: 44 lbs of potatoes will feed 100 people (plus the ingredients needed to make the sauce). So you would need appx. 180 lbs of potatoes plus the rest of the ingredients. Here is a link to the recipe for 100 servings, which would have to be quadrupled for 400: http://members.tripod.com/~lotsofinfo/H78scallopedpotatoes.htm
I hope this information helps. Please let me know if you need clarification of anything I've provided. Thanks for using JustAnswer.