Been asked to make a 3 tier wedding cake for a friend. She wants 3 tiers, wants 40 servings. She wants to save the top tier, so the top tier would not be included in the 40 servings. Only the bottom 2 tiers are needed to feed 40 people. What size round pans should I use? Thanks.

I don't think you can do 40 servings realistically in a 3 tier cake.
I'd do a 6/8/10 but using Wilton's chart you'd get 62 pieces out of the bottom two tiers. A 10" gives you 38 pieces. So if she was fine with that many, you could make the 8" a dummy cake. I wouldn't go any smaller than 6" on top. A 4" is a really small cake.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm