C3 is clearance. C4 is tighter than C3 and it is for more precision applications. I'm not sure that tighter clearance will work better on high RPM under load because it will take more heat and worst lubrication, in addition thermal expansion with close clearance will load the bearings + any debris in the precision bearing can damage it even worst. They use these C4 bearing in the transmissions because transmissions mechanism needs less bearings freeplay, all transmission bearings immersed in oil, so they have less thermal load and better lubrication environment, and there are no debris in the transmission, plus transmission bearings do not have extra load unlike the wheel bearings.