I think it's a hardware issue, not a software one. B-4 is an earlier prototype with a less advanced neural network. His processing ability is just not on the same level. He got all the software he could ever need when Data did the memory download, but his hardware just wasn't able to handle it.
For the sake of debate - In comparison, then it could argued that humans are not as advanced as Data (processing, memory storage/retrieval), at out current state of evolution. Maybe then: Less advanced neural network + Segments of Data's memories, software = Closer to Human.
Geordi did make the comment "Maybe it'll just take longer for him (B4) to incorporate the download" - paraphrasing
So there were a few "out clauses", if the B4/Data storyline/issue was to have been revisited in a future Nxt Gen movie.