You have no real alternative to 4diamond. Double (which would be for take-out) would put too much emphasis on Spades - after all you might force partner to bid 3spade on a poxy four-card suit. 4diamond, vulnerable, must show a good hand, after all.   North raises to 4heart, East surprises you with 4spade (you haven't asked him to join the party) and South bids 5club.

What now?.

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