Fourth Suit Forcing?

If a partnership bid three suits and then the fourth, for example, 1 spade, 2 clubs, 2 diamonds then 2 hearts, the fourth suit (hearts) is a conventional bid. It shows at least 11 points and is forcing. No particular holding in hearts is guaranteed, although it is still possible that responder has a heart suit.

What are the replies that opener might make? Simple rebids of either of his suits i.e 2 spades or 3 diamonds or a rebid of 2 no-trumps are not forcing. Neither is simple preference to one of responder's suits. A raise of the fourth suit is natural when a 4-4 fit is still possible. In the above case the fourth suit was bid at the 2-level (i.e. 2 hearts). The fourth suit at the three level e.g. 1 heart, 1 spade, 2 diamonds, 3 clubs is forcing to game asking opener to describe his hand further.

The greatest potential for misunderstanding in fourth suit forcing is when the fourth suit is bid at the one-level. If the bidding is 1 club, 1 diamond, 1 heart and 1 spade, the fourth bid is natural and forcing for one round. It shows atleast four spades but does not show substantial extra values i.e the responder will have less than eleven points.