^Well, obviously there's no one set of rules that applies equally to all SF universes, except the rule that they can make up whatever rules suit their purposes. They're free to contradict each other -- or reality -- all they want, but a given fictional universe should be consistent with its own precedents. So the only examples that matter are the ones that come from the specific fictional universe you're talking about. And the precedent of Lal shows that in the Trek universe, downloading one positronic brain's memories into another positronic brain will not automatically bring the personality along with it. And what we've seen of B-4 to date -- at least onscreen and in the novel continuity -- is consistent with that precedent.
True, but it would probably have caused a lot of problems for Data if he had multiple personalities of different gender in his positronic brain. Which would be dominant. We've seen evidence already of this is in 'The Schizoid Man', a normal computer could only hold Graves intellect but Data could hold his personality too. I realize circumstances were different though and Graves was Human but it's not too much of a stretch. B-4 was pretty empty headed to begin with leaving room for a Data personality in there somewhere.