One thing is clear you are dropped into what seems to be the middle of a story and Lucas was assuming the core science fiction/comic book audience would recognize the convention of this being something like an old time serial in the Flash Gordon vein. The opening crawl up is a direct take from those old time movie serials of the 1930s 40s and 50s. The first time you come across episode IV is in the draft script in the Art of Star Wars book which was published in 1978. It says on its title page
STAR WARS
EPISODE IV A NEW HOPE
FROM THE JOURNAL OF THE WHILLS
BY GEORGE LUCAS
REVISED FOURTH DRAFT JANUARY 15 1976
LUCASFILM LTD.
This was most likely the shooting script or very close to it since it was in 1976 that most of the movie was shot.
Lucas said he didn't think the movie would be huge but hoped for a moderate success. He probably didn't add the episode IV when it first came out since he was expecting it to be a one shot movie deal and thought why confuse things more and went with the simpler Star Wars title by itself. That title page in the Art of Star Wars however suggests he had that title in his mind well before the movie opened in 1977 The Journal of the Whills part who were supposed be I guess the narrators of the whole saga is an idea he never developed further for the movies.
I don't think the Episode IV was in the original release, but good luck finding a print or video without it. It's one of those scenes that fans can't be sure they remember, and it was the first sign that Lucas wouldn't stop revising the movies just because they'd already been shown in theaters. That's his right; it's his baby. But I'm not buying the DVD boxed set until after he's dead, because that's the only way I'll know he's done messing with it.
That title page in the Art of Star Wars however suggests he had that title in his mind well before the movie opened in 1977.
According to the interviews on the 1st (of God knows how many) revised VHS release, he had eps. IV-VI completely mapped out before he sold the studio on one movie. As you mention, he had no confidence he'd get all three made, so he rewrote it to get some of his favorite bits into the first movie.
For example, he liked the idea of a wookie so much that he made Chewbacca one. When he got to what was originally supposed to be the wookie scene, in Jedi, he chopped the wookies in half and called them ewoks.
My other favorite bit from those interviews is Lucas' summary of the story -- A farm boy, a wizard and a pirate team up to storm an impenetrable castle, fight an evil sorcerer and rescue a princess. Get past the sci-fi trappings, and it's the oldest story in the book.