In the chess world, there are many players who like to torture weaker players with the 4-move checkmate. It is an old and tired gimmick that I'd like to see disappear.
When we see the signs that our opponent is trying this quickmate, we must defend against it effectively. If we do not, even if the mate is not realized, the opponent who failed to defend will usually find him/herself in a loosing position within the first 4-10 moves of the game. This is how it works:
There are a couple other variations as well:
The signs of the four move mate, then, are clear: The Queen and Bishop (and as is the case in the 7 move mate, Queen and Knight) pointing at the e6/e7 square. A key square in defending, however, is the e4/e5; if these squares are not properly defended, the recipient of this attack will see massive damage:
In addition to the Four Move mate, this sequence of moves often casually becomes another threat: the "Fried Liver" attack, which is the Knight and Bishop attack:
A few key moves is all that is required to stop the four move checkmate: