After winning the first trick with the ♣A you can count 4 possible 
losers : ♠A, ♦A plus two ticks in Clubs. 
One losing trick too many for making your contract of 4♠.
  
Wrong play 
Normally when playing in a trump contract you draw 
the Opponents' trumps immediately.  But if you do that this 
time the Opponents will win the first trump trick with the 
♠A and immediately play 
and win the two tricks in Clubs and the ♦A and you are one down.
  
Right play 
At trick 2 you must play the ♥A
from Dummy, on which Declarer plays his singleton ♥7.
  
Declarer (W) 
♠ - K Q 10 4 
3 
♥ -  ..  
♦ - K J 10 3 
♣ - 7 6 ..
 | 
♥7
 |    
 |  < ♥A
 | Dummy (E) 
♠ - J 9 2 
♥ -  .. K Q 
3 
♦ - Q 9 5 
♣ - .. 9 3
 |  
| 
 
 When Dummy now leads his ♥K (at trick 3) Declarer can 
no longer follow suit and therefore can discard a 
losing Club from his hands. When Dummy follows up 
with leading his ♥Q (at 
trick 4) Declarer discards his last remaining Club. 
  
  |  
Declarer (W) 
♠ - K Q 10 4 
3 
♥ -  ..  
♦ - K J 10 3 
♣ - .. .. ..
 | 
♣7
 |    
 |  < ♥Q
 | Dummy (E) 
♠ - J 9 2 
♥ -  .. .. .. 
3 
♦ - Q 9 5 
♣ - .. 9 3
 |  
 
Now it is safe to draw the Opponents' trumps, for 
when  they  gain the lead by winning with the trump ♠A they can only cash the 
♦A. For if they play 
another Club Declarer will be able to ruff it in his own 
hand.  Declarer therefore should make 11 tricks in 
total, one trick more than required for his 4♠ contract.
 
Declarer play in Trump contracts
  |    |