There used to be a small difference in that area, but the newer Gen 3 and Gen 4 barrels are the same. (They share the same part number and everything.) Besides, those half circle cutouts only serve to retain the recoil spring while the slide is disconnected from the slide. Once the slide assembly is installed, the recoil spring no longer rests on that cutout on the barrel. You can still use a Gen 4 recoil spring assembly with an older Gen 3 barrel with the smaller cutout. And vice versa, with a Gen 3 recoil spring assembly and a Gen 4 barrel with the larger cutout.