Originally posted by aginis this one is pretty famous please don't post the solution if you've already seen it.
oh and partial solutions will be considered incorrect.
1. Kg5 Re5+ (has to or the pawn gets promoted)
2. Kg4 Re4+ (as above)
3. Kg3 Re3+ (as above)
4. Kf2 (rook cannot now check and be safe... a move like Re5 hoping for Rf5+ is trivially defeated by f8=Q after Re5)