Thanks for the answer.
So we can say that the episode numbers starting at 4 was already part of a design and at first they didn't add it because they didn't know if the movie would be a success (speculation) and as the movie was a hit and Lucas made good money he was able to make ep5 and ep6 and then add the episode number in the title? (that would be logical from my point of view)
Now another question: when did they got the first tape out for rent/sale in the US? (because apparently in France some tape don't show the episode number in the title)