I have three 686's. 8 3/8", 4", & 3". I also have a Python in a 6". So I think I'm qualified to speak on this one
The 6" is going to give you a little more velocity and will be more tame with full-house rounds. That extra 2" of underlugged barrel will make a big difference when you're shooting flame-thrower ammo.
That said, I'd go with the 4". My 4" is my favorite gun (of ALL my guns) to shoot. It's really well balanced, feels "right" in the hand (for me), and it's still managable with full-house rounds. I shoot my 4" (out of box stock) in IDPA.