East’s bidding was frisky, but she does have a good hand, and West knew that all his high cards were working. 4H looks good: six trump tricks on normal breaks, three spades, and, since South is favourite to hold the king, at least two diamonds. There is one obvious danger: a bad trump break.
Declarer won the spade lead and tackled hearts. When South showed out she won the king and played the seven, covered with the ten and queen. Then she played the king and nine of hearts, losing to North’s jack. The king of clubs was ruffed. If declarer draws the last trump she makes two more spades and the ace of diamonds, but that is only nine tricks – South has two clubs to cash for one down. So declarer played a diamond before drawing the last trump South won and played a club. Declarer ruffed, leaving North with the last trumps but, fortunately, no more clubs. She scraped home by playing a diamond to the queen and a diamond towards hand before cashing spades (if she takes the spades North has a spade winner).
It is better to establish the side suit before tackling trump: declarer should play a low diamond at trick two. South may switch to clubs, but declarer uses every available entry to dummy (including a spade ruff) to ruff clubs, eloping with all her small trump.