Shagrat and Gorbag discuss "going off with a few trusty lads and setting up on their own." Orcs don't like being bossed around, or being considered expendable cannon fodder by their big bosses. But they don't seem interested in being self-supporting: raiding and robbing productive neighbors is their style.

Thus any orcs surviving into the 4th Age would keep up their raiding as long as possible, only considering alternatives if Aragorn's kingdom had enough power and reach to effectively police all of NW Middle Earth. Maybe, like some real history sorta analogs as the Huns or the Mongols, they'd settle into herding or something, though that is hard to see.

Had Saruman repented, would his orcs have followed him, maybe in the Isengard contingent at the Morannon? Would Aragorn have accepted them as allies. Would the ents have let them pass undisturbed. Somehow, I don't think so. More likely, they'd stand by S. in the last defence of ruined Isengard, after Sauron has his victory...