wait just one minute, what exactly is this 'b2 pawn' talk about?...if white to move, I see mate in 2 (push pawn to h7 followed by queen promotion) possibly 3 (mitigate any futile block attempt by the black rook)...
edit: nevermind...I see now the black rook can instead attack, which is also futile but would make it mate in 4...still would like explantion on the 'b2 pawn' however...