1.
[
{B:6BB/6pR/6k1/4K1p1/32 BLACK}
{V:White mates in four.
At first there doesn't appear to be anything at all unusual about this problem, but ...;g5g4;Correct;Incorrect.;Any hint would give the solution away completely, so just for once I am keeping quiet!}]

2.
[
There is a small trick involved in the problem (I am allowed this once in 50 puzzles).]

3.
[
Doubtless many of you thought that the solution was 1 Bxg7 g4 2 Kf4 g3 3 Bf8, followed by 4 Rh6 mate, but alert readers will have noticed that the condition is 'White mates in four' and not 'White to play and mate in four'.]

4.
[
Suppose that White is to play in the diagram. Then Black's last move was either a king move, or a pawn capture h6xg5. If the king has just moved, it must have come from f7, h5 or h6, but in every case there is no way that the white checking piece could have arrived on its present square. On the other hand, if ...h6xg5 was the previous move then the bishop on h8, which must be a promoted pawn, could only have arisen as a result of g6xh7 and h7-h8B. But then there would be a white bishop on h8, and black pawns on g7 and h6, so there would be no way for a white rook to reach h7.]

5.
[
It follows that the diagram position is impossible if White is to play. Therefore, Black is to move and the solution is...]

6.
[
{S:g5g4}
1...g4]

7.
[
{S:h7g7}
2 Rxg7+]

8.
[
{S:g6h5}
2...Kh5]

9.
[
{S:e5f4}
3 Kf4]

10.
[
{S:g4g3}
3...g3]

11.
[
{S:f4f5}
4 Kf5]

12.
[
{S:h5h4}
4...Kh4]

13.
[
{S:g7h7}
5 Rh7 mate.]