I was under the impression that B-4 would just become Data after he's absorbed all of his memories.
I find it very weird that people have that impression, because Nemesis made a very clear point of establishing that B-4 couldn't do that. Data tried to give B-4 his memories to increase his potential, but the effort failed because B-4's brain was just too primitive and incapable of any real growth. It'd be like trying to run a modern open-world MMORPG on an '80s game console -- the processing power and memory just aren't there. Data even said outright that B-4 was incapable of growth, that he would never become more than he was -- as a deliberate thematic parallel with Shinzon, another mirror-image character who was doomed by his inability to grow and change. The final scene with B-4 singing Data's song wasn't meant to say that B-4 would become Data (although of course it could've been retconned that way if there'd been another sequel); it was just a final hopeful note to end the movie, to suggest that at least some echo of Data survived, however faintly.