As I recall the story, Lucas had written a basic story outline for the story that encompassed the OT, the prequels, and possibly what was going to be the sequel trilogy before he scrapped the idea. He was writing it as one movie, and quickly realized there was just no way he could cram it all in. He decided to start at what would become Ep. IV, not because he had the OT completely fleshed out and knew what he wanted to do with it, but because space movies weren't selling at all at the time, and he figured if it took the most likely route and only ended up being a single move, the Ep. IV story was the most self-contained and could easily work. It is pretty obvious when watching some of the documentaries on that era that he had the general, big pieced of the story in place, but figured out most of the details way later as he was going along.
That being said, none of that is really relevant. The fact of the matter is that, whether Lucas created Ep. IV in a complete vaccuum, had all nine movies completely written beforehand, or something in between, when he did the SE's and the prequels, he unfortunately ruined a lot of what he had previously created. Whatever his motivations were, he hurt his reputation and the inegrity of the stories he created. That's really all that matters.