As you can see, this would not be a success. Well, not exactly. Now exit with a club. A normal trump fit consists of a combined holding of at least eight trumps in declarer's hand and dummy. But it is well known that lesser holdings sometimes produce an excellent contract.
Moysian fits are delicate contracts.
With only a slight superiority in trumps declarer must time the play carefully to keep things under control. The bidding was sound, though aggressive. North's jump to 4 was a splinter bid showing a good heart fit and a singleton or void in clubs. The A was onside. The rest looked easy, as declarer saw no remaining losers.