It's very well known and very easily proven. There's still existing copies of the 1977 film that do not have the subtitle a new hope or the episode number. What you is the 1981 re-release that had it added to match episodes 5 and 6. Lucas didn't put a 4 on the original release, because he didn't know if there was going to be more movies approved.

He had a series of movies in mind, but chose to start with the middle as it was what he considered the most important part, and the part that stood out to him as the most potential for a stand alone in the event that he couldn't get the others made.