Why 4... Nxb4 is not played in Evans gambit? 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.
It comes down to flexibility. In most cases, Nb4 will end up in the same position, as you said, but Bb4 give you at least two more common alternatives, and a couple less common ones.

 [FEN ""]

 1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Nb4 5. c3 Nc6 (5...Na6?) 6. d4 exd4 7. cxd4


Compare this with this line where black can play Ba5, Be7, Bc5, and even Bd6 have been played a fair amount of times. It is, generally, favorable to make your opponent worry about more options.

 [FEN ""]

 1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Bb4 5. c3