A story can be presented in any order, but why start with an episode IV?
George Lucas already knew that there were earlier materials, he had the basic story worked out in advance (sources disagree on exactly how well developed they were, though). The reason he started with Episode IV (according to himself) was that it had more potential for commercial success and, since he was a relatively unknown card at the time, therefore easier to get a studio to accept. Apparently he was right.
I think the general, most basic idea behind it all was...
In medias res or medias in res (into the middle of things) is a Latin phrase denoting the literary and artistic narrative technique wherein the relation of a story begins either at the mid-point or at the conclusion, rather than at the beginning (cf. ab ovo, ab initio), establishing setting, character, and conflict via flashback and expository conversations relating the pertinent past.
Likely original to the oral tradition, the narrative technique of beginning a story in medias res is a stylistic convention of epic poetry, the exemplar in Western literature being the Iliad (9th c. BCE) and the Odyssey (9th c. BC), by Homer.
I think Lucas consciously picked this kind of opening to create what he believed would be an epic (in all senses of the word) movie.