I would like a mix of cherry and other sizes. Figure indeterminate growth.
I'd go with five. You can find many sites that'll tell you that you can space tomatoes as close as 24 inches, 6 to 9 plants in a 4' X 4', but mine always come out stunted when I do that. One near each corner, and one in the middle should give your indeterminates plenty of room to grow and fruit. Five happy plants can give you a lot of tomatoes.