[JanisW]

Our rule set for 4NT is
1) if 4NT is needed as a natural limiting call, it is that.
2) if 4NT is needed for take-out, it is that
3) if it is none of the above, it is RKCB

Typical examples for 1) are

• 1NT-4NT of course, but also something like
  
• 1♠-(2♣)-2♥-(P)
  2♠-(P)-3NT -(4♣)
  where we have bid 3NT to play.

Applying these rules:
1 through 4 is natural/quantative
4 is a bit tricky though, P might easily be 7-5 in the black-suits and we should not have a singleton club for our bidding.
So he might reply as if 4NT was Ace-asking. I'd take a 5♦-reply as a 7-5 hand with 3 Keys...As I said it is a bit tricky.

5 clearly is RKCB for ♥

regards
JanisW

[Tramticket]

1-3 Quantitative
4 Natural, sign-off.

5 Is the interesting one and depends upon partnership agreement. I have played differing methods with different partners:

- 4NT is RKCB for partner's last naturally bid suit (hearts). This is our current agreement.
- 4NT is RKCB for partner's first naturally bid suit (spades). In an Acol context, the 1♠ opener only promises a four-card suit, but the 2♥ rebid now shows 5-4 in spades and hearts - it is reasonable to bid 4NT for spades.
- 4NT is natural and invitational (too strong to bid 3NT, not strong enough to bid 6NT). Note that I don't play 2-over-1 game force, so I can't bid 2NT as forcing with this invitational hand. If wanting to investigate a heart slam I would use 4th suit forcing first and then 4NT (a 3♥ raise would also be non-forcing). I no longer play this and I didn't particularly like this method when partner persuaded me to play it - but with hindsight, it was a playable method with some benefits).

I am also aware of some who play 4NT in this sequence as ordinary Blackwood and only use RKCB once a suit has been explicitly agreed. (Not my choice at all).

[pescetom]

 Tramticket, on 2018-July-16, 10:47, said:

1-3 Quantitative
4 Natural, sign-off.

5 Is the interesting one and depends upon partnership agreement. I have played differing methods with different partners:

- 4NT is RKCB for partner's last naturally bid suit (hearts). This is our current agreement.
- 4NT is RKCB for partner's first naturally bid suit (spades). In an Acol context, the 1♠ opener only promises a four-card suit, but the 2♥ rebid now shows 5-4 in spades and hearts - it is reasonable to bid 4NT for spades.
- 4NT is natural and invitational (too strong to bid 3NT, not strong enough to bid 6NT). Note that I don't play 2-over-1 game force, so I can't bid 2NT as forcing with this invitational hand. If wanting to investigate a heart slam I would use 4th suit forcing first and then 4NT (a 3♥ raise would also be non-forcing). I no longer play this and I didn't particularly like this method when partner persuaded me to play it - but with hindsight, it was a playable method with some benefits).

I am also aware of some who play 4NT in this sequence as ordinary Blackwood and only use RKCB once a suit has been explicitly agreed. (Not my choice at all).

Playing 2/1 GF I remain happy to leave #5 undefined in an uncontested auction. It makes no sense as quantitative here and would be precipitous as RKCB, why renounce discovering more (at least a control) first? If the opponents are competing then it's a different matter and is clearly RKCB in hearts.