Daily Puzzle Submissions! *Be Involved*

Okay, here's a puzzle I came up with. Not sure if it is a hard one or not. Take a look.
White to move and mate in 4.
seems to me this is (an easy) mate in three. Rb8 (protected by the bishop). k to 7th rank. Rc7 (protected by the bishop). k to 6th rank, but can't get beyond the bishop in these two moves. Rb6# the king cant take the bishop, can't take Rc7, and can't move any furter towards the rank of pawns.
Sorry if someones all ready seen this (or its wrong). This refers to 270#
So everybody have to be involved because it says *Be Involved*.
not really. i just like to make puzzles
ps that was good. can anyone on chess.com tell me these two things!_if i only have a king left"how many moves does my opponent[s] have to checkmate me? it would be obliged if someone could answer that for me!+what is the passing pawn move+im a begginer at the game and i need to know these things . also what is a fork?in chess; obliged wudge.
A fork:
The pawn forks the knight and bishop (see left)Also, your opponent can take as long as s/he wants to mate you
I don't know how to explain a passed pawn but it isn't important - E
thats what i get, its mate in 1. Nd6
I get the same thing