Fourth Suit Forcing should be part of any player's arsenal. I wouldn't even think of it as a convention. It is really a necessity.

Why? Because jumps by responder in a previously bid suit are NOT forcing. In modern bidding, each bid below (on the second round by responder) is INVITATIONAL:

 

OPENER	RESPONDER
1	1
1	3

OPENER	RESPONDER
1	1
1	3

OPENER	RESPONDER
1	1
1	3

 

In each case, Responder is inviting. What if he wants to force? Then, he would bid the 4th suit at his 2nd turn.

 

Fourth Suit Forcing is used by the responder to establish a game force. So, after, say, 1-1-1, a bid of 2 is the 4th suit. Any time the auction starts: 1 grape-1 apple, and opener bids a 3rd suit, responder can bid the 4th suit (artificially) to set up a game force. 

I recommend that the 4th suit is 100% forcing to Game. Example: After 1-1-2, responder holds:

 AQ432  
 4  
 Q32  
 AKQ2

. He bids 2 to set up a game force. The bids says absolutely nothing about the suit. It could be 432, it could be AKQ. It could be a singleton, it could be a five-card suit.  If opener has the 4th suit stopped, he will usually bid notrump. Both players assume the 4th suit is unstopped until either player bids notrump. 

Opener's 3rd bid (after 4th suit forcing is used)

The opener bids naturally following a 4th suit forcing bid. He should have a stopper in the 4th suit in order to bid notrump. He is allowed to take a preference to responder's first suit with honor-doubleton.

Example hands for opener after:  1-1-2-2:

53
K2
KQJ87
A1076
  Bid 2NT (hearts stopped)
Q5
54
KQJ87
AJ74
  Bid 2 (this does not guarantee 3-card support)
53
Q
AK765
KQ1065
Bid 3 to show 5-5

Opener can jump raise responder's first suit with three cards and a maximum, such as:

 KQ107  
 K32  
 AQJ43  
 2

(after 1-1-1-2, jump to 3). 

Notes:

Only the responder can use 4th suit forcing (never the opener).

It has to be the 4th bid of the auction.

Fourth suit forcing is typically not used after a 2/1 GF response. The 4th suit would be natural in that case (1-2-2-3=clubs). 

Fourth-Suit is NATURAL and non-forcing by a passed hand (P-P-1-P-1-P-2-P-2 = 5-5 or longer in /). If the opponents have made an early double, 4th suit forcing is still on. If they overcall, it is off.

Fourth Suit forcing is alertable.

Some players use 2 as "4th suit forcing" after 1-1-1 in order to leave 1 available as natural (this is just a matter of partnership agreement).

Fourth Suit forcing is not used after a 1NT rebid by opener (new minor forcing is used).

If you enjoyed this article, you might consider Larry's webinar on the same subject found HERE.

For Advanced players: See also XYZ.