But "Countdown" didn't explain it, beyond him "returning". It was the STO novel "The Needs of the Many" that did.
And the fate of the B-4 is seemingly further clarified in this "Star Trek Online" short story in the recent issue of the "ST Magazine":
I was sure data said something to spock about b4 & memory engrams after spock makes his resurrection comment. But maybe I'm mistaken?
From Memory Beta's description of issue #2: "Data asks to see Spock. Spock notes that he and Data have not seen each other since before Data's 'resurrection'; Data, however, believes 'return' to be a more apt description since it denotes the fact that his neural nets were successfully imprinted onto the existing programming of B-4. Spock notes that he and Data share a unique experience..."