I was using a mix of 3s and the multiflow ones until Caitlin was about 6 months. When we changed all teats to 4s I found that she didn't dribble as much milk down her chin (dunno why?!?!) and it's easier to let her feed herself when you haven't go to worry about making sure she has the teat the right way.
I guess the girls will learn not to suck so hard on the 4s
*waffle waffle*
so yes, I'd go with 4s