Not withstanding the differences in fight choreography in the standards of the 70s from the PTs, I honestly believe neither Vader nor Obi Wan were truly taking their duel seriously in Episode 4. I am rewatching the fights right now and even I can't help but witness Vader is not using his full skills to fight Obi Wan. I mean he shown more ferociousness and accuracy in fighting Luke in the Empire Strike Back (and this was when Luke still was a noob with a saber). And Vader wasn't trying to kill Luke. Its obvious Obi Wan is holding back either especially since he let Vader strike him. But even considering his fall to the Dark Side, I find it hard to believe Vader was genuinely trying his hardest against Obi Wan and believed he truly beaten his master this time. I mean Vader must have seen the force being used when Obi Wan died without leaving a corpse. Furthermore Vader was using far more techniques against Luke than with Obi Wan. Even Obi Wan told Vader if he dies, he will become more powerful than he can ever imagine, proof that Obi Wan was not going at fullest. I'd go as far as saying young Luke was portraying far more athleticism, agility, and attacking from more angles than either Obi Wan and Vader did in their duels when he lost his arm in ESB.