The most difficult part of the game of bridge is defence. In order to defend accurately, you must use all the clues available and then apply logic to find the winning play.
Here you are East and South opened 3 which was raised to 4 . Your partner leads the 3 playing fourth best. You win the ace, and declarer plays the 4.You have two likely tricks and the trump ace how do you find another trick? Could a club switch be right? From the bidding you can place Declarer with 7 hearts and from the lead he has three diamonds. That leaves room for only 3 black cards. Any club loser will be pitched on the top spades.Where is the best chance of a trick ? The 10. How can you stop declarer ruffing his diamond loser in Dummy? Play a trump but which one? If you play the A and another he is in control, and may throw his diamond loser on good spades . Play the 4. Now when partner wins the King he will return a second trump, and you will cash the 10 for the setting trick.
Tip : Use the opening lead and bidding as clues to find the winning play