the 4 is a good companion knife - handles small chores and some big ones, great for dressing out and segmenting game, etc...
the 5 is a moose of a blade for it's size. has been described as a sharpened prybar by some. It'll survive just about anything you can throw it at, but will not be as good at fine tasks as the thinner bladed, flat ground esee4.
that said, whichever one feels best in your hand will be the one you use the most and both will do what you want/need them to. the 3 & 4 share handle size, the 5 & 6 share handle size.
I have a 3, and large enough hands that I'm going to be careful getting used to it so I don't create a hot spot where my hand overlaps the back of the scales. (at some point I'll either cut up some cumaru or try making my own micarta and build a set of oversized scales to fit me)