i've done some thinking and came up with a big theory..
Ben Kenobi is the clone of Obi-Wan Kenobi..
in episode 2 they say clones can be altered physically, or mentally...erase memories etc..
im guessing anakin does indeed kill obi-wan, has him cloned (being he was a father figure to him) and hopes to turn him to the darkside and he fails
now since in cloning u can erase memories..it probably explains why he "doesnt remember owning a droid" in episode 4
and my biggest peice of evidence is when Luke asks Ben..."you know him?"
Ben replies... "of course i know him...HE'S ME!"
correct me if im wrong...if someone were to ask u if u knew someone with ur name..wouldnt u say I'M HIM?...and not "HE'S ME?"
just a theory