This was a UI case at the club tonight. I was west and opened 2
♣ showing the majors. It is not that common here. North bid 3
♣ alerted and explained as spades and diamonds quieting my p's bidding. South bid 4nt (not alerted) and North passed!
I asked what 4nt meant and was told keycard. So i lead a heart and they made 430. Since they had an easy 6
♣ on, we didn't bother complaining. Besides, it was only a club night, not the bermuda bowl.
Anyway, I got to thinking and decided that if I was directing and had ruled this I don't think I'd let them play in anything less than 6
♦. It sounds like I would have been justified in my reasoning.