Data downloaded his 'essence' into B-4 in 'Nemesis' which eventually asserted itself over the primitive B-4 personality and was deemed fit to basically resume Data's life. Worf apparently went back to serve in the Empire after becoming dissatisfied with the way things were going, and married Grilka. I think this is all 20+ years after 'Nemesis.'
So Worf went back to the Klingon empire again. And B-4 is now Data? IS any of this explianed in any way or what?
Quantum mechanics allowed all of this to happen. That's the easiest explanation out there.