*sighs* the reason why Lucas named it 4 5 and 6 is because he wanted it to look like it came from a bigger story. Originally he had just written short notes about where the Original Trilogy characters came from but he never intended back then to make a 1 2 and 3.
It's a style thing. Lucas wanted the audience to feel as if a lot had gone on before we get to this point so the story he created would look much bigger than it was back at the time.
When he did decide maybe it'd be nice to make a 1 2 and 3 he waited until the technology was just right before he started so he could take advantage of all the things that didn't exist back in the days when he made 4 5 and 6.
Remember "The Star Wars" was originally one big script which was then split up into A New Hope, Empire Strikes Back and Return of the Jedi. But the prequals only existed as notes of the characters past.