I like 4 as opposed to three as I like to have people doing different roles - Something like 2 fighters, black mage, white mage. If you did this with three members then you'd have two who would be weak..so..it's better with four, like in FFIX.

Although I liked FFX, being able to switch members in and out, but it made it kinda too easy. Oh it's got a shell - switch to Auron, it's flying - switch to Wakka, and so on..but it was fun.