As far as your machine, I'm not really sure. Maybe one of the resident Guru's here can help you with that, I've noticed they are quite helpful.

The only thing I can see, is some who run 5 toons I believe use 6 gig of ram, but I could be mistaken on that also.

I know I am using 2 gig and running three fine. (sadly it's the max I can go with this old MB) so 4 should be ok.

Stephen