Blackwood or Gerber?
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Quantitative, Normal Blackwood, RKCB, Gerber, Splinter or what?

  When is 4 asking for aces? When is 4NT asking for aces? When is 4 a cue bid or a splinter? When is 4NT quantitative? What is 4 after a transfer? and after Stayman?  
  These are all questions that a regular partnership should be able to answer, but it's less easy for non-familiar pairs. In general my recommendation is that 4 is only Gerber if partner's last natural bid was NT. Lets look in detail. It is assumed that you play RKCB where applicable and that you do not play Kickback.  
West East  
1 2

4 is a splinter, agreeing 's. It is could be either a singleton or void. 4NT is RKCB. Some play exclusion RKCB here but I don't really see why.

4 4NT
1 1NT

What is 4 over the strong 3 ? If East had a weak hand or a limit raise for 's or 's, he would simply bid game. Thus 4 can only be a cue bid agreeing 's. Responder has a suit with insufficient values for an initial two level response. A bid of 4NT here or a subsequent 4NT bid by either is thus RKCB for 's

3 4
1 4 A splinter or Swiss, according to partnership agreement.
1 4 A splinter or Swiss, according to partnership agreement.
1 4NT

Normal Blackwood. This cannot be RKCB for 's as then East would first bid a forcing raise (maybe Jacoby 2NT). It is not quantitative, as East would presumably first bid a suit. It must be a strange hand.

3NT 4NT 3NT is gambling. This 4NT is not Blackwood, opener has exactly 1 ace. Responder has a good hand and simply requests opener to bid 5 of his suit and responder will take it from there.
3NT 4 3NT is gambling and 4 is pass or correct.
3NT 4 3NT is gambling and 4 is a conventional bid asking about shortage.
2 2

RKCB for 's. With a big hand in support of 's, East would have splintered, cue bid, bid 3 or bid RKCB on the previous round. Of course it is unwise to use 4NT as Blackwood when minor suits are trumps and my preference is to use Kickback (so 4 would be RKCB for 's here), but that's a totally different topic which is discussed in the Kickback link.

2 2NT
3 4NT
And let's have a look at the situation after a 1NT opening: -

Remember what I said about Gerber and partner's last natural bid? Here's what I mean: -

West East   | West East  
1NT 4 Gerber | 1NT 4NT Quantitative.
1 2   | 1 2  
2NT 4 Gerber | 2NT 4NT Quantitative.
1NT 3

Assuming that you play that East's 3 is a slam try, then West's 4 is a cue bid agreeing 's and East's 4NT is RKCB for 's.

4 4NT
  Transfer Sequences |   Stayman Sequences
1NT 2

Gerber (RKCB?). Partner's last natural bid was 1NT.

| 1NT 2 Ace (key card) ask, so
2 4 | 2 4 Gerber (RKCB?)
1NT 2   | 1NT 2  
2 4NT Quantitative (5 's) | 2 4NT Quantitative (4 's)
1NT 2

This time, 4NT is RKCB for 's. West's super accept of the transfer has set 's as trumps. East re-transfers to get West as declarer and then uses RKCB.

2NT 3
3 4NT  
1NT 2

4NT is not RKCB for 's here, it must be quantitative. If East had a hand where he can investigate slam in 's with minimal support from partner, he would have started with a slam interest bid of 3 over 1NT (if that's what you play 3 as). Some players (including me) play 4 as RKCB for 's in this sequence rather than Gerber.

2 4NT
  Note : What do I mean by Gerber(RKCB?)? I play 4 as Roman Key Card in these situations and rather than call it RKC Gerber I prefer to say that 4 is RKCB.  
  Note also that in the sequences like 1NT - 2 - 2 - 4 and 1NT - 2 - 2 - 4 then 4 is normally played as standard ace ask Gerber, I believe that RKCB is to be preferred but you would have to agree that.  
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