That Ain't Right... #4

Question

Here's a small head-scratcher for you all today.

In the above position, it is White to play and mate Black in four moves, assuming that Black plays their best moves.

However, there is a catch—there are two solutions here. One is legal, and the other isn't. The task is to find both possible solutions

State which ONE rule of chess is being broken (minus the illegal position of the White pawn on c1) in the Illegal solution.

Clarifications:

1. "Legal" chess moves means that it is accordance with the FIDE laws of chess. So an illegal solution would contain a move that is illegal under that rule set.

2. All of the pieces move normally.

3. Pieces cannot occupy the same square.

Have fun solving!

Answer 1

I'm going with the legal version being:

1. Ra8 b5
2. Rh8 b4
3. Rh2 b6
4. Rf2#

And the illegal one being:

1. Ra8 b5
2. Rh8 b4
3. Rh1 b6
4. O-O#

Where the illegal move is castling after the king side rook had already moved or been taken.