Why did George Lucas start at episode 4?

Answer 1

Because IV-VI (4-6) are often called the "Origional Trilogy" George Lucas thought of that idea first, and made I-III (1-3) later to tell the story of what happened beore episodes 4-6. 4-6 would be sort of incomplete without 1-3.

George Lucas wrote all six episodes at once. when deciding which one to make first he went with episode IV because it was by far the cheapest and easiest to make.

Addition by SJ Commander:

While that may be part of the reason, another reason was that George Lucas wanted to make Episode I first, but he decided against making it because he wanted to make the giant, sprawling metropolis Coruscant as good and realistic as possible, and the CGI techniques and matching programs and computers weren't there yet. So he decided against messing with his grand vision and made Episode IV first because it required less grandiose CGI techniques.

Answer 2

The location for the beginnig of the film was aboard a ship in orbit over the planet Tatooine

Answer 3

George Lucas wrote the Star Wars Movies as just one movie at first. He thought if he started at Episode 4, it had a better chance of being a success.