^The movie made it pretty clear that B-4 was very simple and incapable of the kind of growth and learning that Lal showed. I mean, that was the whole thematic point behind the character.
DATA: The B-4 is physically identical to me, although his neural pathways are not as advanced. But even if they were, he would not be me.
PICARD: How can you be sure?
DATA: I aspire, sir, to be better than I am. B-4 does not. Nor does Shinzon.
Lal is completely different because she learned and grew readily. B-4 asked a lot of childlike questions but showed little sign of understanding the answers or being able to assimilate new knowledge or insight.