Someone tell me if I'm totally in the wrong here, but you're probably better off just selling your 6 inch revolver and buying a 4.2 version. It'd probably cost you more to have one modified by a smith.
It can be done. The question then comes down to, can it be done economically. My guess would be no. Unless the smith has a 4.2 in a drawer and offers a swap + install charge you'd be better off putting up a wtt in EE or a wtb then sell yours.
gl
Well since S&W doesn't list a 4.2 inch version of the 586 I'd say your search is destined to fail.
If you have the sort of good taste for blued guns that I have and simply can't live with that "silvery stuff" then you're likely looking at $250'ish to have the barrel shortened and a new front sight put on. The key is to find a smith that is also a machinist instead of just a parts technician.
I tried to have barrels shortened on 2 revolvers. End result butchered revolvers sold at below fair pricesss. Buy new.
I haven't been able to find any 4.2" 586??? It's either 6" restricted or 4" prohib.
Heers A couple pics ........



























