Personally I think a 10G is far too small for 4 apisto's, even 3 is pushing it a bit in my opinion. As a harem fish the females need to have enough territory, caves etc, and also enough hiding places to get away from the amarous males. Also the females will fight for territory, quite vigorously sometimes. The 20G is the better option but you already have a full stocking in that tank. You're going to have a pretty busy tank if you do use the 10G.