on the thx vhs versions of the trilogy there's a 3 part interview.
lucas basically says (and the rough drafts really reflect) that 456 were the first idea. he then made up the back story and when he knew he could make a film.
he originally was going to make the entire 456 story line in one film but it was too much. so he did the first act (ep 4) and had to extend it and play around with it. he never thought he was going to make any sequels. he didn't think star wars would do good at all.
that's why when he got to ep 6 he had originally planned to have kashyyyk (the wookiee world) very primitive but chewbacca was already shown as a pilot and very intelligent. so he changed it around to endor and as he says "cut the wookiees in half and called them ewoks". you can even see ewok is almost wookiee spelled backward.
while he was making the ot he conceived the prequels most important elemnts like anakin being vader etc. so by the time he made ep 5 he had the prequels in his mind.
it's also vey cool that he's coming back around to his original ideas in the prequels, like mace windu was a dude's name in the rough drafts and utapau, mustafar and the wookiee world kashyyyk will now be in ep 3.