Why has s a new Hope always been episode 4?

I heard that when the Empire Strikes back was released, they actually called 'A New Hope' Eisode 4. However, aparently Lucas didn't plan to do prequels at that point, so why was it called episode 4 if there was no plans for prequels. Surely that's ironic. Any help would be apreicated.
Thanks.

It was actually a later edition.

Indeed, it was only when ESB was released that ANH was renamed Episode 4, and Lucas decided that he wanted to do prequels based on Vader's story.