Trade with someone who wants a 4" gun. I have a nice 8" Trooper Mk III, but prefer the 4 & 6" barreled ones. Mine has a mount for scopes/red dots and does not have the rear sight. Perfect for a hunting pistol unless one wants to use irons. I have thought about trading or selling it simply because it doesn't get shot much, but I may not.
Surely their are others out there with 8" guns they would trade. Just a thought. Surely cheaper than buying a barrel and paying someone to install it.
As dfariswheel posted, the barrel length has nothing to do with what loads it can handle. Trooper Mk III's are very tough guns that can handle any reasonable .357 loads.