It is a widely accepted term used frequently in Golf, other golfers know that when they hear fore the should be wary that a ball may be coming their way, and they'll duck because getting hit with a golf ball will hurt.
FORE - Meaning lookout ahead
It's another way of saying "watch out" According to the site about.com: Fore" is another word for "ahead" (think of a ship's fore and aft). Yelling "fore" is simply a shorter way to yell "watch out ahead" (or "watch out before"). It allows golfers to be forewarned, in other words.
Fore.
It is "Fore", not "four", hence not "five" either. The "Fore" comes from the old use of "Fore-caddies" who were down the fairway looking for the golfers ball coming down the fairway from the tee. The golfer would yell "Fore" telling the "Fore-caddie" that the ball was on the way. Now, of course, "Fore" means "Watch out! Ball is coming your way."
The word "fore" can be a noun, adjective, or adverb. All have a common definition meaning front or beginning. It is a rarely used word. The word "for" is a very common word and is a preposition. It is generally used as a function to indicate purpose.
As today players many years ago had caddies, and each caddy had a "forecaddy", the forecaddy would assist the caddy. The forecaddy would stand out in the fairway and wait for the balls to be hit from the tee, so they could be found. As soon as the player hit the ball, the caddy would shout "fore", a shortening of the word forecaddy, to let him know the ball was coming.
a fore caddy
The danger call is "Fore" ... best thing is to duck or crouch down as far as possible.
To call people incase of an inmergancy, or to use to communicate with your friends
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Regarding Boat and ships: Aft Regarding golf: Oops, call 911
They're not shouting the number four. They're shouting "Fore!" which I believe is just another way to say "Head's up!". Since I don't golf, I don't know for sure.