Very true Ben, but Balboa is older in Rocky 4, and hence may have lost some of his fitness and ability as a fighter as a result. Then again one may argue that Balboa may improve as a fighter as time passes by.
I see your age point, however I think that Rocky IV could beat Rocky III in the ring. Rocky keeps the high agility that Apollo teaches him in III but backs it up by refining his classic raw punching power, which he develops during the most gruelling training montage of all time. He runs up a mountain for fucks sake. Beats running down a beach and swimming.