A good fun post..! I got it first try, but I feel that maartenotton's solution was the most innovated and ingenious..! CUDOS..!

bisshop97, then black's king has to take it (no other place for him to go after the revealed check. then rookb8 has to move to g8, checking the king. The king has to take the rook, so white promotes the pawn, checking the king. The king has to go to g7, so you mate him with you queen to h8. Is that the solution?

How about this: -

1 r h8+;k*r

2 p e8=q+;n f8

3 q*f8+; k h7

4 b g7++

Yusong, what if i take it with:

1...BxRh8 ... i think your move is useless. You will just lose all your major pieces. But anyway Black is losing here

 

Marshal Dillon:

In your sequence, white would play: 3. Qxf8+ Kh7 4. Bg7#. 

Maartenoten: have you checked it? 'cause I did and it still gives me five moves... but maybe I'm missing something:

First of all, after the underpromotion the King goes to f8 instead of g8. If the next move is Nf6+ then black's Knight blocks the rook for one more move with Nd8.

So white's next move would be Bh6+ instead, met by black's Bg7, delaying mate one more move. If instead of using his bishop black uses his Knight, then white's Nf6 would do the work (checkmate), so black uses the Bishop and survives one more move. After this there's nothing to stop the mate in 5: both of black's moves (g8 or Nxg7) are met by white with Rh8#.

A note for all: there's too many boards loading. If we are going to continue to find mate in 4, only post a board if you are sure you've found the solution, or this is going to get really slow to load (at least for me, having only a very humble broadband connecion). I'm sure we're all good for some abstract thinking

Overclocked: It doesn't matter if black goes to f8 its still mate in 4

 

Maartenotten: I like it, but the King still avoids mate in four if he goes to f8 instead of g8 (the famous horse blocks the rook -> one more move)

Ok marshall_dillon i didn't saw that you wrote that

maartenotten, 

 

Please read what I wrote along with the position I showed. I know the knight can block checks along the back rank. 

 

This is a solution:

hey marshall

What if black does:

3. ... Nf8

than mate is not there.

I thought this was it but that stupid knight can foil any check along the back rank.

That's right OC. I found at least 3 mates in 5 but couldn't possibly show all the ways yusong's solution could be foiled but I wanted to show one where he managed to queen the pawn since his solution depends heavily on it. I am still trying to find a way to force mate in 4, though.

    Hey overclocked what if you do 3. Nf6+ instead of 3. Bh6+

 You can still mate in 5:
    3.Nf6+ Qc8 4.Bh6+ Ng7 5.Rxc8#

---

And yes, Yusong's solution is not forced, but not because of marshall's proposal:
    1.Rh8+? Bxh8! (and the game continues for several moves)

 

yusong,

 

What do you do if this happens instead? Your sequence of moves is not forced.

http://www.chess.com/forum/category/more-puzzles

Hey overclocked what if you do 3. Nf6+ instead of 3. Bh6+

this is it, 4 move checkmate

Hmm... what about 5 moves?