Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Nc3/3...Nf6/4. d4

4...d4

The Scotch Variation of the Four Knights has a repertoire of being aggressive & unusual. Some point out that it is unsound, but some exclaim it as a natural grab to the center. While still somewhat organic, if Black plays his cards right, then he can save the lost pawn.