To me they keep Grant, Jackson, Sutton, Hall, and Johnson if you had to pick the 5 RBs/FBs they'll keep on the 53-man roster today.
I think Wynn has definitely shown flashes, but my biggest concern with him has always been his durability. He hasn't been healthy in either of his first two seasons in Green Bay. I do like the fact he has improved his work ethic during this past offseason, but looking at back for back, out of Lumpkin, Sutton, and Wynn, Sutton has by far been the most productive.
I will concede at this stage, Wynn is probably the better pass protector then Sutton, but Sutton offers more versatility being a KR as well as pounding the rock. He seems to have good vision and has done a good job hitting the holes the offensive line and QJ have opened for him.
Obviously there are still a couple of preseason games left to play, but I've always thought the back who could stay the healthiest behind Jackson and Grant would probably be the back they keep on the roster. I suppose you could always carry 4 RBs on the roster, but there isn't a whole lot of wiggle room and you'd probably be cutting someone on defense you'd really like to stash on the 53-man.