What is 4C?

A bid of 4C may carry different meanings. First of all, bear in mind that

• 4C is never Stayman (which applies only to 1NT opening and 2NT opening)
• 4C is never used as an invitation to 5C

There are four main types of 4C bids.

(1)  As a suit
Although relatively rare, 4C is sometimes natural. 

Scenario	Example	Meaning / Remarks
As a flee after opponent's penalty double	... > 3NT > (X) > 4C	A redouble at 3NT or above is not SOS
Weak-4 overcall showing 8+ cards	(1S) > 4C	Not for a 4C opening which is strong!
Competitive bid after opponent overcalls at 3-level	... > 3C > (3H) > 4C

(2)  As a response to a conventional bid
Sometimes 4C is used as a response to a (normally forcing) conventional bid by partner.

Scenario	Example	Meaning / Remarks
A response to 2C opening	2C > 4C	9 controls
A (strong) response to takeout double at 2-level	(2S) > X > 4C	Rarely used as it bypasses 3NT
A response to takeout double at 3-level	(3H) > 3NT > 4C
A (negative) response to a stopper-asking bid	... > (2H) > 3C > 3H > 4C

(3)  Help suit slam try
After reaching a fit in majors, a bid of 4C is generally used as HSST. It is also used for such purpose in some other cases.

Scenario	Example	Meaning / Remarks
After reaching an explicit major fit	1D > 1S > 3D > 4C 1H > 2H > 4C 1NT > 2H > 3S > 4C
After an unusual jump	1D > 2S > 4C	A spade fit is deemed to have been reached
After a 2H/3H response to Stayman	1NT > 2C > 2H > 4C	This cannot be Gerber since responder should either find a heart fit or have a 4-card spade suit to show
After a weak opening (without interference)	2D/2H/2S > 4C 3C/3D/3H/3S > 4C

(4)  Gerber
While the original Gerber is used after an NT bid, many other scenarios have been added in which a 4C bid after a suit bid is still used as Gerber. In general, any 4C bid which does not fit into one of the above three categories, especially when both sides know that there is no major fit, should be regarded as Gerber.

Scenario	Example	Meaning / Remarks
After NT bids	1D > 1S > 2NT > 4C
4C opening	4C	Opener shows 28+ balanced hand
After a 2D/2S/3D/3S response to Stayman	1NT > 2C > 2S > 4C 2NT > 3C > 3D > 4C	Gerber has a higher priority than HSST
After both sides know there is no major fit	1C > 2C > 3C > 4C 1C > 2C > 4C 1C > 2D > 3C > 4C 1D > 2C > 4C 1D > 2C > 3C > 4C 1D > 2D > 3D > 4C 1D > 2D > 4C 1H > 2C > 4C 1H > 1NT > 2C > 3C > 4C 1H > 2S > 3H > 4C 1S > 2H > 3D > 4C 2NT > 3C > 3H/3S > 4C
Some sequences after 2C opening	2C > 2D > 3C > 4C 2C > 2D > 3S > 4C
Others	1D > 2D > 3C (!!) > 4C 1H > 1S > (2C) > 3C (!!) > 4C	A stopper in clubs is required in such case

Finally, the following lists some scenarios which should not occur:

Bidding sequence	Reason why it should not occur
1A > 1B > 4C	As responder only shows 6+, opener should not have enough strength to try slam immediately
1M > 4C	With a strong hand, responder could use two-over-one to show strength
1NT > 2S > 3C > 4C	After minor suit Stayman, responder must show shortness with a strong hand
1NT > 3C > 3D > 4C	After minor transfer, responder must show shortness with a strong hand