Sbodge said:

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)

If you would ask Lucas today he would perhaps say it was part of the design, but the truth is that the 1977 Star Wars was a self contained story and film, if for some reason they had included a title number back in '77, it would've been Episode I and definitely not IV, there's even storyboards depicting the planned crawl for The Empire Strikes Back that says Episode II, somewhere in the writing process of Empire, Lucas realized and decided that he could make the first film be the fourth in a big epic saga.

These episode numbers weren't really a big deal back then, some were obviously a little bit confused by seeing Episode V in the crawl on Empire in '80 but most people just took them as an homage to the old Flash Gordon serials which the films emulated. Oh, some time have passed since the last one, that's the way I saw it back when I first saw Empire in '80, not thinking I had missed three episodes. ;) I had actually no idea about planned sequels/prequels then.

The title Episode IV - A New Hope first appeared in the published script of the film in '79. That published script is a little bit different to the real actual shooting script. But fans who read it and those who were members of the fanclub etc. knew about the future title change, a few years before it actually materialized on film prints in '81.