4NT Invitational

To bid 4NT after a NT bid or 4th suit bid is invitational and ask partner to pass with a minimum hand, and show aces with a maximum hand. It is important to have agreement of when 4NT is invitational and when it is Blackwood. 

• If you play Gerber in notrump sequence, 4NT is always invitational when 4C Gerber hasn't been used.
• If you use just Blackwood, 4NT is invitational only directly after NT bid and 4th suit.
• If you use RKC-Blackwood you need to be careful. If last bid show suit, you have agreed to a trump or you are in a cue-bid sequence, 4NT is RKC-Blackwood. If not 4NT is invitational.