Ive been learning the theory for this opening but cant find anything if black chooses this line.
You probably mean 4...Nxb4.
White will play 5.c3, and there is probably nothing better than 5...Nc6; so it boils down to being the same as 4...Bxb4 5.c3 Bc5.
[FEN ""]
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.b4 Nxb4 5.c3 Nc6
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In case of Nxb4, white also has the option to simply 0-0 and ignore the knight, since e5 is hanging. It's not good for white to castle when Bxb4, because of this missing threat. – d4zed Oct 27 '20 at 14:05