was viable with a "fault" -- he was a fully functional sentient, learning, even emotional being, but he was a psychopath. B-4 had the equivalent of a severe learning disability.
Julianna said the other three prototypes were "lost." She made it sound like they died. I've always assumed that meant they suffered cascade failure just like Lal (and Rayna Kapec, most likely), that they couldn't cope with emotional conflicts without collapse. The fix Soong attempted to solve the emotional-conflict issue with Lore resulted in his psychopathy, his lack of empathy for others; there's no emotional conflict if he never cares about anyone but himself. So Soong just deferred the whole emotion question and built Data with that capacity suppressed until he could achieve a viable fix.
B-4's limitations are on a very different level, a lack of cognitive ability. He wasn't "lost," he didn't collapse; he just plain didn't work. So he seems like an earlier model to me.
And as I said, I think the only way B-4's existence can be reconciled with Julianna's account is if he was made before the three prototypes she knew about.