Tough question for me. I own 4" and 6" 586s.
The 6" is without a doubt the most accurate .357 I own. I love it!
However, my 4" was sent by its previous owner to a very good gunsmith who turned it into an outstanding combat revolver. I use this gun for IDPA.
I also have a 686 that's been fitted with one of the surplus 5" barrels that Gun Parts Corp sold a few years ago. Over all, I really think this is the best all around barrel length.
Like I said , tough call. But if it came down to it, I'd keep the 4".