Yes, but when he became that new fighter he beat Clubber in 3 rounds, and actually did it relatively easily. Clubber got in some good shots and there were a few spots where Rocky was in a lot of danger, but for the most part, it was Rocky from beginning to end.
He had to change to beat Clubber, but that's only half the picture. The other half was that he was mentally checked out for the fight, didn't take it seriously from day 1, didn't really want it, Mickey begged him not to do it, etc. It wasn't like a fully motivated and fully trained Rocky, who had the full support of his trainer and put everything he had into, went in and got killed. It was a shell of a man who didn't want to be there, was stressed to the max, panicking over Mickey, etc that Clubber killed. Of course Apollo's training is what helped him win the rematch, but it was ALSO the fact that he overcame his demons, fully trained and prepared, was fully motivated and gave it his all. He was at peace.
For IV, it was that same Rocky, only better. It was the absolute best version of Rocky there's ever been, who had accumulated all those new skills from Apollo, plus all his old skills from Mickey; who knew literally everything there was to know about boxing, was in the best shape of his life, trained the hardest he's ever trained in his life, was the most mature he's ever been, was maxed out in every single category, primed, hyper-motivated, laser focused, went in to fight Drago and STILL got his ass absolutely massacred for 90% of the fight.... that tells me Drago was his best/greatest/toughest opponent ever.
