Fourth suit forcing
If three suits have been bid and responder then bids the only remaining unbid suit then that's the fourth suit, of course, and it's forcing to game. It doesn't promise cards in that suit, 'fourth suit forcing' is just a convenient way to show a good hand.
Let's take a simple bidding sequence where the opposition are silent.
You have enough high card points to bid game, but which game?If partner had three card spade support, you would like to be in 4♠. If partner had a hold in diamonds, you would probably like to be in 3NT.
If partner had neither of the above, you might want to be in 4♥ or maybe even 5♣. Your dilemma is that you are too strong and have the wrong shape to bid either 3♥ (only 2 hearts) or 3♠ (shows 6+ spades). Without fourth suit forcing, you have to bid 3NT and cross your fingers. Even if partner has a diamond hold, like K5, it is much better they are declarer to prevent the opening leader playing ♦Q squashing the king under the third player's ace.
No problem, South bids 2♦, the fourth suit and partner must bid again.
South bids 2♦, North shows a diamond stop and South raises to 3NT.
Responding to 4th Suit Forcing.
What happens when your partner makes a 4th suit forcing bid? What do you bid? The answer is you make the most useful bid you can.
- support partner
- bid no trumps
- show extra length in your second suit
- rebid your first suit with none of the above or with extra length in that suit.
Let's look at four South hands after our second sequence and see the choices. Remember our North hand, the same each time.
Bid 2♠, showing three card support for your partner's suit. You cannot bid no trumps with a singleton in the unbid suit. Again, you have already told partner you have 5 hearts and 4 clubs. Hopefully, you and your partner will have at least an 8 card spade fit between you.
Bid 3NT. You have a good diamond hold and have already told your partner you have 5 hearts and 4 clubs. Note you showed 5 hearts when you bid 2♣. Had you only 4 hearts and 4 or 5 clubs, you would have opened 1♣.
You cannot bid no-trumps or support spades. Your partner knows you have 5+ hearts but the one thing they do not know is that you have a fifth club. Therefore, bid 3♣ showing your second suit clubs is a 5 card suit.
You cannot bid no trumps and have no spade support for partner. Your partner knows you have at least 5 hearts and 4 clubs. Help! What do you do? Partner knows everything! Don't pass, ever! The answer is that you rebid your opening suit, 2♥. This may either show a 6th heart (Partner only expects 5) but could also say, as here: "I have nothing else to say, partner. You know my hand shape, roughly, and I can neither support you nor bid no trumps."
After 2♥, North is too strong to bid 3♦ as that bid shows a long diamond suit, little else and around 10 hcp. So, North checks out for the best game by bidding 3♣, 4th suit. South shows their 5th heart with 3♥ and North raises to game as they have found a fit. 4♥ is the only making game, losing just the ♠A and two club tricks.