1. Pawn to E3
2. Queen to H5
3. Bishop to B5
4. Queen to F7 and Checkmate...
That is assuming your opponent didn't move a Knight to C6, or F6, and didn't move a pawn to A6, C6, D6, E6, F6, or G6 at any point during those four steps.
Such a combination is rare but oh is it so sweet to be able to say "Checkmate" after 4 moves.