As the name suggests, Fourth suit forcing is a “forcing” bid which means your partner has to bid again and tell you something more about their hand.
It is used by the responder when three suits have already been bid (opener bid a suit, responder changed suit, opener changed suit again). At this point a bid of the fourth suit on its own tells the
opener nothing useful and would be unhelpful, so it can be used as a conventional bid meaning “tell me something more about your hand”.
Its most common use is asking partner if they have a stopper in the fourth suit so that you can make a No Trumps bid.
Using Fourth Suit Forcing
If you changed suit after opener’s first bid and opener changed suits again, you may not know all you need to know to make the best contract. You know opener has 5 cards in their first suit and 4 cards in their second, but suppose you hold a 5 card major?
When opener bids a third suit after a change of suit response, it is not always possible to determine the best contract immediately. This is often the case when responder has a 5 card major suit.
Imagine that you are South and this is your hand:
The bidding has gone as follows:
It’s tempting to bid 3NT. You know opener has at least 12 points, so you know that between you have enough points for game. (You have at least 26 points as your hand has 14 High Card Points.)
But before you make that bid, just think about partner’s hand. They might hold 3 hearts, so you might have an 8 card fit in a major suit. If they do hold 3 hearts, then they only have one club, making 3NT a less attractive contract.
This is where we use Fourth Suit Forcing. We bid the fourth suit – in this case Clubs at the 2 level. We aren’t telling partner anything about our clubs, but we are asking them to bid again and tell us something more about their own hand.
If partner has 3 hearts, this allows them to bid them..
Partner has showed that they have 3 hearts so we can bid 4H.
Here partner has showed they either have 6 diamonds or that they simply have nothing else to show. They don’t have 3 card support for our hearts, so we bid 3NT.