Recently I have been playing the Evans Gambit as many times as I can and for the most part, black plays 4...Bxb4, but someone played 4...Nxb4 and I wasn't sure how to continue. After the following moves below, how should I continue with the Evans Gambit as white?
[FEN ""]
1. e4 e5
2. Nf3 Nc6
3. Bc4 Bc5
4. b4 Nxb4
5. c3 Nc6
6. d4 exd4