If both Defenders follow suit to the 1st heart trick, Declarer follows up with playing the ♥K at trick 2.
If both Defenders follow suit once more they will have only one more heart left.
In this case : Declarer at trick 3 leads the ♥4 to Dummy's ♥Q (capturing Defender's last remaining heart) and the ♥10 will secure the 4th trick.

But if at trick 2 (as shown above) South had a singleton heart, discarding a card from another suit (here ♣3), Declarer must at trick 3 now play ?