The full hand is shown below. The key to the hand is getting a heart ruff and this is only possible on a heart lead or diamond lead (when partner will switch to a heart). On a trump lead, which is what I made, a heart switch is too late. A spade lead removes your entry for the subsequent ruff however a spade continuation may break the contract - this is complex but, I think, that if East takes the diamond ace on the first round declarer can eventually throw three hearts on the diamonds and still have a trump to overruff the heart king; however if East ducks and then returns a heart he will get a ruff but not make the diamond ace and the contract still makes, presuming declarer divines the position (it was their best player as declarer so reasonable chance).
Anyhow I think Alex was a little unlucky to find 4C a reasonable contract and I think North was extremely lucky to find partner with working cards as South would have bid the same with no useful values.
Although I singled this board out, in truth the match was lost on many other boards and not this one. Interestingly the double of 4C guaranteed a winner, since if we had beaten an undoubled contract of 4C we would have needed extra boards. However Alex's double was not due to the state of the match, as this was our sixth board of the final set and we had done reasonably well to this point.
Board 42, Game All, Dealer East, IMPS
North
K
AK
QJT93
KJ984
West East AQJ5 964 QJ8763 5 72 A8654 2 A653
South
T8732
T942
K
QT7
West North East South
Pass Pass
1H 2NT* X** 3C
3H*** 4C X All Pass
* minors
** ability to penalise at least one minor
*** weakness