As Dennis pointed out, the depth of the flutes has a big impact. One really has to be able to take another cut after the flutes are gone to smooth out the inevitable chatter from turning the flutes of. If the flutes are too deep, it is a waste of time. Going from a 4 to a 2 may not be enough to remove the flutes (you don't say who's 4 or who's 2).
While re-contouring a barrel is not a big deal, by the time a gunsmith sets his lathe up to contour, removes and replaces the barrel, recrowns, polishes and beadblasts, he will be lucky to get away with two hours. Check with your local machine shop or auto dealership to see what 2 hours should cost. Regards, Bill.