He didn't. The numbering was added later.
Star Wars on Wikipedia
That link explains that the numbering didn't show up until The Empire Strikes Back and then was added retroactively to A New Hope (along with the subtitle).
Here on the The Empire Strikes Back page you can find a succinct description of the writing process for Empire. Lucas originally labelled Empire "Star Wars II". He did a hand treatment, from which experienced writer Leigh Bracket did a first draft. That first draft does not have Vader as Luke's father - it has Obi Wan killing Luke's father. It was only in rewriting that draft after Bracket died that Lucas came up with the idea that Anakin was Luke's father. It was then that much of Vader's backstory was invented and the IV -> V numbering shows up.
Lucas likes to claim precognizance of the whole thing. But if you look at the documents that were produced around the time of A New Hope's and Empire's productions, they suggest he was making it up as he went along. The truth is he had a lot of help writing two of the first three movies. He basically plotted them and left much of the writing to other people. And some of the major plot elements in Empire and RotJ didn't actually come from him, but from cowriters who never really got credit for it.
This is one possible reason why the original series was so good, and the prequels were so bad. Lucas didn't have the same sort of help on the prequels and his legend was such at that point that no one would stand up to him and say "George, this sucks."