Partner leads spade J to your king and ace. (You don't know why partner led into declarer's four-card suit but let's assume he will be sensible from trick two on.)
Declarer plays four rounds of diamonds and will then cash a spade queen. Plan your defense.
Arend

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P-P-1♣-1N
P-2♣-P-2♠
P-2N-P-3N