You can't be sure if B-4 is really the prototype. He could also be a bad copy made for only one purpose: Infiltrating the Enterprise.
In Nemesis, it was never made clear if he was really the prototype or rather was only meant to "get through as the prototype".
And I believe we won't find out before they decide to do a new TNG movie.
Btw:
I thought Nemesis was a nice idea, but with an asshole as director and with the idea to kill Data, I simply hate that movie (except the Romulan stuff

). It was more or less the day Star Trek died for me (partly). And while I'm writing this, GL is killing Star Wars... thank god I switched my hobby to living history.