In these, the B4 matrix sacrificed himself to force the Data matrix into accepting B4's body and step for the Federation in a time of need.
I thought I'd read that B-4's matrix was downloaded into some other system and thus survived, freeing Data to use his body without the ethical problems.
I loved the little tidbit that Spock pointed out to Captain Data that they both had returned from the dead, in a fashion. Given that they are both the breakout characters of their respective shows, it's symmetry is fitting.
I think you could make the case that Worf is a bigger breakout character than Data. Generally "breakout character" implies one whose popularity is unplanned and greater than expected, and that's more true of Worf. He wasn't even part of the original proposal, was added as an afterthought, but became a central member of the ensemble and literally "broke out" by being spun off to another show after TNG ended. Data was certainly popular, but he was always expected to be a central player, the inheritor of Spock's role as the outsider commenting on human nature.
For that matter, you could say Picard was a breakout character in a way. While he was always the lead character, it was assumed he'd take a back seat to Riker as the action and romantic lead, and it ended up being more or less the other way around. And nobody had any idea just how popular Patrick Stewart would become in America, going far beyond TNG.