I'm pretty sure that all 686s have a lug.
Not so. There was a stocking dealer special a few years ago that had a 5" barrel with a partial lug, a red Hi-Viz front sight, and combat finger-groove stocks. There's also the currently-produced M686 SSR, which has a tapered slab-sided barrel and forged lockwork like the M686 no-dash through M686-4. However, you're correct about 95%+ of M686 production; most have full lugs.
FWIW many S&W enthusiasts consider the -4 to be the high point of the "traditional" M686 because it came drilled for a scope base and was available with the 7-shot cylinder, but it still has the old-style forged lockwork and hammer-mounted firing pin. -4's command more money on the auction sites for this reason. However, all of them shoot about the same, and if you dislike the ILS, it can easily be disabled (instructions can be found on this site and elsewhere).
IMHO there's no reason to turn down any particular dash number from a functional standpoint, although you'll need a gunsmith to drill the frame if you want to mount a scope on a -3 and earlier model.
Ctjoe, I suggest cross-shopping the M586 if you don't feel you need stainless finish. They're generally a bit cheaper and are functionally the same, although the gun's production run ended before the introduction of the 7-shot models, so all M586s are 6-shot except for a few pricey and sought-after Performance Center special editions. You could also look at the M620, which was basically a discounted 7-shot M686+ with a 2-piece partial-lug barrel. All M620s have the ILS.
FYI S&W sold a few .38Spl-only M586 and M686 South American and European contract overruns. They also sold a Performance Center M686 in .38 Super. These guns are quite rare and command high prices on the auction sites simply because of their rarity; there is no reason to choose one of them over a run-of-the-mill .357Mag model unless you are a collector.
