Fourth suit forcing

If an opening bid, partner's response and opener's rebid show three different suits, then if responder bids the fourth suit in the second bid, it is known as a fourth suit forcing (4SF) bid. It does not promise anything in the fourth suit but is generally game-forcing (unless there is a clear misfit).

The basic 4SF sequences are:

• 1C > 1H > 1S > 2D
• 1D > 1H > 1S > 2C
• 1H > 1S > 2C > 2D
• 1H > 1S > 2D > 3C

Note that responder may or may not have support for opener's major (which could be shown in the next bid). Opener responds according to the following priority list:

(1)  Show 3-card support for responder's major if any (since responder may have a 5th card in the suit).
(2)  If not, bid NT with stopper in the fourth suit.
(3)  Otherwise rebid opener's minor (does not promise extra length).


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On the other hand, the two 4SF sequences below are more special:

• 1D > 1H > 2C > 2S
• 1D > 1S > 2C > 2H

These sequences may be intended as 4SF, or may show 5-5 (former case) or 5-4 (latter case) in the majors respectively. (In the latter case responder should have close to GF strength as bidding will normally go on to 3-level.) The priority lists are as follows:

Priority	After 1D > 1H > 2C > 2S	After 1D > 1S > 2C > 2H
1	Bid 3H to show 3 hearts	Bid 2S to show 3 spades
2	Bid 3S to show 3 spades (deny 3 hearts)	Bid 3H to show 4 hearts (deny 3 spades)
3	Bid 2NT to show stopper in spades (deny 3-card major)	Bid 2NT to show stopper in hearts (deny 4 hearts or 3 spades)
4	Rebid 3C or 3D otherwise	Rebid 3C or 3D otherwise

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It should be noted that 

• 4SF is OFF for passed hand. In this case bidding the 4th suit is natural (and a NT contract is not preferred).
• 4SF is ON even if a suit is only implied by not explicitly bid, e.g. 1H > (X) > XX > 2C > 2D as the XX shows a spade suit.
• Opponent's suit does not count for a bidding sequence to qualify for being 4SF, e.g. 1D > 1H > (1S) > 2C is not 4SF.