Sorry, I have to respectfully disagree with Kellie.
A child of 4 can wipe their own butt. Just stop doing it for him. I NEVER wiped my girls once they started using the toilet (age 3). I think I talked them through it once or twice and watched, but that's it. At first there were times their underwear was a little dirty, and I pointed out they needed to do a little better wiping. They improved quickly with more practice. A little mess at first is well worth the independence gained from doing it themselves.
I still remember my neighbors and some friends with preschoolers who would yell and scream for their Moms to come in and wipe them. And the Moms would come running. They would even yell at my house for ME to wipe them. I'm sure if I started out by doing it for my own, my kids would demand it too. As soon as I nicely told our young guests that I only wiped kids in diapers, and that kids big enough to use the toilet in my house were big enough to wipe themselves, they complied. At my house, they wiped with no problems. At their house, they yelled and demanded Mom do it. I also remember another little girl at 4 came over for a playdate and got VERY upset after pooping because she could only wipe with "wet ones." This I didn't know, and we didn't even own any wet wipes.
I would monitor him, but make sure he does it himself. Even if it's not the best job, he'll improve. If he flat out refuses, natural consequences. It takes more time for him to change underwear more frequently and put dirties in the laundry. Stock up on a few more packages of underwear. More baths, and possibly he will be a little sore and itchy for awhile. Gently make the connection back to wiping. He'll figure it out.