4... Nxb4 is never played on chesstempo database for +2200 players games, being Bxb4 the most popular move.
I don't find any significant change after capturing the pawn with the knight unless after 5. c3, black can play Na6 supporting an eventual c5 to block the a3-f8 diagonal.
If 5...Nc6 it looks the same position as 4...Bxb4 5. c3 Bc5
Stockfish evaluates 4... Nxb4 as =:
Why doesn't this move appear on databases is what I am wondering.