I don't think you can go wrong with either length. In terms of quality they are equal. In terms of characteristics, the 4" is easier to carry, faster to draw and aim, while the 6" will give slightly better velocity, a little less felt recoil, and potentially be a bit easier to shoot more accurately, although I thinkbo th are equal in terms of inherent accuracy.

I have a few 586s and a 686 in both lengths, and to me, the 6" looks and feels more right, but if I carried it I'd go with the 4", or even 2.5"unless I was going open carry. But seriously, the 686 is probably one of the finest revolvers available in .357, in any length.