Fourth Suit Forcing
For more advanced players the bid of the fourth suit is a request for a stop in that suit. If partner has bid two of the suits and you have bid one suit, then if you wish to play in no-trumps with guards in the 4th you should bid accordingly. However you may believe that 3NT is the contract but need partner to bid it holding a guard in the un-bid suit (with you not holding a guard). The suit can of course be natural, and at the 1 level is always considered to be so. The bid of the 4th suit is forcing for one round , and at the 3 level should be forcing to game. You may bid the 4th suit having a stop but needing a better guard (with possibly a slam in mind). Except at the 1 level the bid of the 4th suit should be alerted. The fourth suit can be bid either by opener or by his partner. A bid of the 4th suit above 3NT is likely to be a cue bid, not a request for a guard, but is not alerted.
What are the responses to a fourth suit bid at the 2/3 level
1. If the responder holds a guard, then NT is bid at the appropriate level
2. The fourth suit may be raised to the 3 level showing a 4-card suit and holding a 5-4-4-0 or 4-4-4-1 hand. Do not raise to 4 level because partner may not hold a biddable suit
3 If responder does not hold a guard in the 4th suit, then in order, re-bid your suit especially a major, or give support (3 card) to partner’s first suit