Take a look at this hand.
West opens 1S, North passes, East bids 2S. South overcalls 3H and North raises to 4H.
West leads 5s - as declarer, what is your plan?
West's lead is very revealing. She does not have two touching spade honours. If she had, she would definitely have led one. This marks East with As and probably Qs.
Now, let's do some simple arithmatic: you and your partner hold 22pts between you; the opponents hold 18pts. If East holds As and Qs, that leaves only 12pts for West.
Therefore, West holds Qh and Qc. As declarer, you must not play for the drop in trumps, but finesse West for Qh, and you must not finesse for Qc - your only hope is that West leads clubs, or Qc drops.
Playing with this knowledge, you will succeed in 4H. Without the thought involved, you would fail.