Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Nxe5/3...Nxe5/4. d4

4. d4

White has unwinged the main line of the Irish Gambit, known as the Chicago Gambit. Black has 2 moves:

4...Ng6

The main line. White either plays 5. c4 or sometimes plays 5. e5 to continue the offensive.

4...Nc6?!

The Kann Variation. Most computers are not fond with this as the offensive push continues with 5. d5! to destroy Black's systems.