Budapest - Rubinstein variation
1. d4 Nf6 2. c4 e5 3. dxe5 Ng4 4. Bf4
White pushes the queen's pawn to d4. Black develops the knight to f6. White advances the c-pawn to c4.
Black plays the king's pawn to e5. White captures on e5 with the queen's pawn. Black transfers the knight to g4.
White develops the dark-squared bishop to f4.