Um.... 4. ... g5 and then the next turn 4. ... g4?
Sorry, not possible.
Anyway, the From's Gambit may or may not be unsound. On one hand, black loses two pawns. On the other hand, he has the centre.
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I've got this book which says that the From's gambit is unsound but I lost 2 games against it in rapid games (30minutes).
I've got the following move order:
1.f4 e5 2.fxe5 d6 3.exd6 Bxd6 4.Nf3 g5 5.d4 g4 6.Ng5 f5
What do you think is White's best move or plan here?