Sbodge wrote: So we can say that the episode numbers starting at 4 was already part of a design

I wouldn't make that assumption.

Before Betamax and VHS there were 8mm and 16mm prints of Star Wars which were available for purchase, sometime in 1978 or 1979. They did not have the episode title. Here's an example of the 16mm:

One of the first tapes was the 1982 Library Rental Betamax:

http://theswca.com/index.php?action=disp_item&item_id=22522

This version (as it was released after the re-release) has the Episode IV title.

Why foreign versions (which were released... later) do not have the Episode IV title, the current guess is that... well there is no definite logic. Each language track may have been linked to a certain print and they may have gone back to those prints to do the next home video version. (which in 1984 would have been the first) Only later when there was the interest would they upgrade to match the latest version with title and episode number.

English 2006 GOUT : http://www.youtube.com/watch?v=QoMQKumx7Zo

1984 CBS/FOX German: http://www.youtube.com/watch?v=r3THFc9ocbI (no episode)

1984 CBS/Fox Spanish: http://www.youtube.com/watch?v=ETli-fE7Ha4 (no episode)

But yet when Star Wars premiered in 1984 on US television it had the Episode IV.