Revision 2362569 of "Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. d4/3...exd4/4. Bc4/4...Nf6" on enwikibooks

4...Nf6 is a two knights defense in a Scotch gambit. White has now two sound options: the obvious 5. e5 and 5. O-O.
In the first case 5. e5 white goes for maintaining a strong center and an open game with chances for both sides. 5. O-O gives away the pawn, and while taking it might seem risky due to 6.. Re1 it's perfectly sound, and white will only get his pawn back.