I think they are the same height, the 2ml is a 22mm wide tank and the 4ml is a 24mm wide tank. Pretty sure that's the only difference but I don't have them in front of me.
im not 100% sure but usually larger capacity tanks are all the same size except for the glass which are bubble so wider not taller so as long as you buy the correct glass for that tank it should fit with no issue.
on the give away and comps section you need 20 posts but for the bargain basement its 100 posts.