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.
The convention applies only from the two level up: at the one level there is still room to show a suit naturally. After 1♣–1♦–1♥, a bid of 1♠ is natural and shows four spades, so the fourth suit forcing bid in that sequence is 2♠.
BidMaestro also uses it only when responder’s first answer was at the one level. After a two-level first answer (1♥–2♣–2♦) responder has already shown the values to keep bidding, so the next bids are natural: a raise, a new suit, or notrump.
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. Once three suits have been bid, a responder with 13 or more high-card points, a balanced or semi-balanced hand and a stopper in the fourth suit bids a natural 3NT straight away, unless there is a fit to show first (three-card support for opener’s major is shown before notrump). 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.
