Though I would tend to agree with steve's rule referred to by inquiry in respect of always (except when over NT bid by partner) treating 4NT as blackwood, all absolute rules admit of exception (or should that be, almost all absolute rules admit of exception?
In this case I thought -- rightly or wrongly -- that given 3NT had been a signoff bid (that's what I thought, anyway), we were unlikely to have slam (based on my own hand). And if as some suggest here my 4♦ bid had suggested slam interest to partner, then I had made a mistake from which the best recovery was to pass 4NT.

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