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).