danah, checkmate in 4 moves is possible. but black king has to move exactly as listed. if black king changes the move then its not possible. So, its not a general solution as the previous one. white 1. Nf5 g3 (check) black 2. Kf4 [/Ke3/Kg3] [ if Ke5/Kd3 checkmate in next 3 not possible] white 1. Nd4 f5 black 2. Kf4 g4 white 1. Qa1 d4 black 2. Kg4 f3 white 1. Qd4 e3 (check mate) but this is the best case scenario. there are lot of moves Black king has to escape.