The only point of 4...Nxb4 is to set a trap already noted in comments
to two of the answers:
[FEN ""]
1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. b4 Nxb4 5. Nxe5? Qf6 6. d4? (6. O-O Qxe5 7. c3 Bd6 8. f4? Qc5+) Bxd4! 7. Qxd4 Nxc2+ 0-1
Interpolating 6 Bxf7+ doesn't help; the best White can do is bail out with
6. O-O, but that's still not nearly enough compensation for the lost Knight.
However, if White plays the normal 5. c3 instead, Black is basically forced
to play Nc6, transposing to the Bc5 variation of the main line
4...Bxb4 5. c3 — so 4...Nxb4 is no better than 4...Bxb4,
and may be worse if you prefer 5...Ba5 in the main line, or if
White turns out to have a better move than 5. c3 (perhaps 5. 0-0,
or an immediate 5. d4 as xaisoft suggests in a comment to
Andrew's answer).