I won't support Data's return unless they come up with something better than his programming overwriting B-4's, which would basically be Data killing his own brother so that he could live again. He'd never do that.
Data didn't kill B-4 in Needs of the Many. B-4 volunteered for an experiment to attempt returning consciousness to Data. During the experiment, the possitronic net began to collapse because it couldn't handle 2 personalities. Data tried to delete himself to save B-4 but B-4 beat him to it and sacrificed himself so that Data could live. Data tried to stop B-4 from doing so but couldn't.
Data was a helpless party who tried to save B-4. The cause of B-4's death was the experiment leading B-4 to heroicly sacrifice himself for his brother.
And why keep B-4 around in the universe? He is useless because his program is inferior. Data provides much more promise.