I like how once the rook is pinned only whites king and knight can move. So after 1. ... bc5 (rook pin) white will have a very hard time preventing the black King from going to h4. Although I have no idea which direction the pawns are moving I doubt your engine did to =p
Answer: 1) ... bf4 2) nd2 Re1 3) nf1 Rxf1 4) Kh3 Rh1#.
*Love how all white can do is stall with that knight before Kh3 is FORCED XD.
This is a puzzle I composed. I think it is very difficult, because when I fed it into my stockfish it gave the wrong move at first and the wrong evaluation, about -5.0. I had to begin entering my combination before it changed its mind and evaluated mate in 4.
Of course my stockfish is not amazingly strong, but I am proud to compose any puzzle that an engine doesn't crack instantly.