As both a "new guy" to these forums as well as a new owner of a brand new 617 I picked up just a couple of days ago, I'll weigh in and say that I am extremely happy that I went with the 4".
I've got other 6" revolvers and like them all, but that full lug and little hole in the barrel (that leaves lots of heavy steel to hang out there) made the 6" 617 feel so heavy to me that I knew the moment I hefted it that it was way too front heavy for me.
There are people who like LOTS of weight out front on a target pistol, and I'm probably one of them.
But the 4" felt so much better balanced, while still being nicely "heavy" overall, that I knew the moment I held it that it was coming home with me.
It was the 6" I looked at first because most of my other target revolvers are 6", so I was really surprised that the 6" 617 was so uncomfortable right away and that it was the 4" that felt so much better.
That's my .02 and I'm sticking with it. But hey, what's a guy's first post worth anyway?

At the same time, I may be new here but I'm still on the wrong side of "old", so rest assured that I'm not just some kid spouting off.