Take the East cards here, defending against four spades.
Your partner, bless his cotton socks, fishes out the heart ace for his opening lead, on which you encourage with the 10. You win the next round of hearts and cash a third round as West throws an encouraging diamond seven. What now?
If partner has either top spade, you will beat this come what may, so assume declarer holds the spade ace and king. Based on the bidding, partner does not have room for both minor-suit kings. Therefore, given his suggestion of diamond values, you will not score any more tricks in the side suits. The club finesse is working if necessary.
You had best look for the setting trick in trumps. Lead a fourth heart in the hope that partner can ruff with the spade nine or 10, forcing out dummy’s queen. Because you have already inferred that declarer can discard any diamond losers on the clubs, you need not fear the ruff-and-discard.
This defense will legitimately beat the contract if partner has the 10-nine, but it will give declarer a nasty headache even if West has 9-x or 10-x. On the second round of the suit, declarer will have to decide whether to finesse against your jack or play for the drop, knowing you have three more hearts than your partner.
Partner ruffs with the spade 10, and declarer overruffs in dummy. Now, when he calls for the spade eight from dummy, you must be sure to play small without a flicker. Since partner has the spade nine, declarer now has no chance to make.