Ok now, you are wrong.
As Black, I wouldn't play c2xb1. I would promote the pawn to Knight to give a check... It extends the play by 1 more move. I already indicated it in the last reply. Read it carefully and analyze it. You Can not force mate in 4 moves. The white king is susceptible to check by the pawn promotion either to queen or knight where ever he goes...!!
Unfortunately , you are wrong.
Read the move correctly. Draw it on a paper and you will see that I am right. This is a
forced mate. The rook is in front of the pawn. The only LEGAL move you have is to take the horsie. I shall illustrate it with pictures below .
I will post screenshots from my Friends laptop . Right now Im on Linux and its hard to do it in Gimp for me.
Original Position
*img205.imageshack.us/img205/5348/nunn5nh8.jpg
Step 1.
*img244.imageshack.us/img244/1768/step2nn6.jpg
Step 2.
*img244.imageshack.us/img244/1830/step3vk4.jpg
Step 3 - (White only moved in the picture. Now its Blacks move)
*img352.imageshack.us/img352/7990/step4ln0.jpg
Once the rook is in front of the pawn the
ONLY move you have is the Pawn to Take the Horse. Next move Rook all the way to the Kings row which checks King. Game over. This is a
Forced Mate. The reason I call it Forced is cause once I play the opponent has no alternative but playing the moves I listed above.