My 2cents part 2...
I agree he had a basic plot in mind, he would not have been able to write ep 4 without it.
I think what alot of people don't relize is that ep 4 introduced all of the major charaters and alot of special effects which was very important if you wanted a hit movie back in 1977.
What Lucas did in 1977 had never been done before. No one had ever seen special effects like that.
I was 10 in 1977 when I first seen ep 4, it blew me away...
I went to see it 10 more times after that. I had to mow alot lawns to pay for the tickets.