blackshoe, on 2015-March-14, 08:30, said:
The director should explain all legal matters pertaining to the situation before anyone takes any action. If a player knows or suspects that the director has not done so, he should decline to take action until the director completes that step*. However, there is no obligation on a player to know all the ins and outs of the law. That's the director's job.
In this case, once the director becomes aware that he has created this kind of problem, IMO he has no choice but to rule director error and adjust the score under Law 82C.
* Law 9B2: "No player shall take any action until the Director has explained all matters in regard to rectification."
I agree completely and the law quote says it all. As head director at a local ACBL club, this is one topic I seem to continually stress, to not immediately give the option of accepting the IB before stating what would happen.
For this case where 4NT is presumably not natural (so 5NT cannot be substituted without barring partner) I would say (after confirming the meaning of 4NT and 5NT away from the table with the 4NT bidder) "West, if you do not accept the insufficient 4NT bid, then the 4NT bid is cancelled and South will be required to pass or make any legal sufficient bid with his partner North being required to pass for the rest of the auction. If you do choose to accept the 4NT bid, the auction continues with no limitations on either opponent. Do you wish to accept the 4NT bid?"