Conventions in depth
Once three suits have been bid, responder’s bid of the fourth suit is artificial and forcing to game.
Sometimes responder has enough for game but no natural bid that settles the contract: no clear support for opener, no stopper to bid notrump, no long suit to repeat. Fourth Suit Forcing exists for exactly this hand.
The three earlier suits must have been bid naturally, and responder needs about 13 points. The bid says nothing about the suit named; it asks opener to keep describing, typically to show three-card support for responder’s major or a stopper for notrump.
At the one level, 1♣ – 1♦ – 1♥ – 1♠ is natural: a real spade suit, not the convention. There is still room to bid spades naturally, so the artificial meaning applies only from the two level up.
The fourth suit asks opener to keep describing, in a fixed order of priority:
Because the fourth suit forced to game, opener may not pass: one of these three answers is compulsory. Responder then names the game: raising the shown support to four of that major, settling in 3NT once a stopper appears, driving a self-sufficient six-card major to game, or raising a repeated minor to the four level when that is the fit.
Sometimes responder skips the convention altogether. After 1♥ – 1♠ and a second minor from opener, a responder with game values, a balanced hand and the fourth suit stopped bids a natural 3NT straight away. With nothing left to find out, there is no need to ask.
The fourth suit says nothing about the suit named: it forces to game and asks opener to keep describing the hand.
