2 = known entity, stable, simple. Used to be a 2GB limit but not sure now. This was in the early days of g1 rooting and I think it was a limitation of early Android releases.
3 = journaling support (unnecessary imo) no size limit. (in practical terms in this use)
4 = new, not well tested, possibly unstable, journaling, no size limit.
I reckon ext2 but I'm sure having posted this someone else will suggest a reason that 3 or 4 is better. Do some messing about. See if you can come up with any useful findings! I'm all ears!