Artificial indicating that responder has no appropriate alternate bid, remains interested in the potential for a game contract and asks opener to bid again to show additional features (minimally forcing for one round and usually forcing to game).
Opener responds to the fourth suit forcing by (in prioritised order):
- raising responder's first bid suit with 3-card support;
- bidding notrump with values in the fourth suit;
- raising the fourth suit with four cards in that suit;
- making the most natural rebid possible, lacking any of the above.