The best contract is 4♥ by East. This is unbeatable, but requires some text book declarer play. Let's say South leads his diamond. Declarer should win, unblock the top spades and then take two rounds of trumps, first with the King and then with the Ace (or Queen). If the trumps break, draw the last trump and claim. When the second round of trumps reveals the 4-1 split, play dummy's spades. South can ruff and cash two clubs, but declarer can win the return, play a trump to dummy's Queen (drawing South's last trump) and cash winning spades.
I held the East cards - did I find the correct play at the table and not just in the bar afterwards? Unfortunately not, as South found a frivolous 2♣ overcall (at green) to disrupt our bidding, so that we had this auction
| West | North | East | South |
| 1♥ | 2♣ | ||
| 2♠ | pass | 3♣ | pass |
| 3♥ | pass | 3♠ | pass |
| 4♠ |
and my Grandmaster partner did not allow me to play the contract. Quite right too. Against 4♠, North cashed two clubs and switched to a heart. Declarer cashed the top spades and tried to get to hand with a diamond ruff. The bad news was that South had a singleton diamond, but the good news was that he had no more trumps. The even better news was that the run of the trumps squeezed South in the rounded suits and Grandmaster made an overtrick.
Only one declarer made 4♥, and that was due to a defensive error, so I don't think that anyone found the correct play. Meanwhile, 4♠ + 1 was a somewhat fortuitous top.
To defeat 4♠, South needs to overtake the second round of clubs and continue with the Jack, allowing North to discard her heart. Not easy, but South might reflect that his partner would raise to 3♣ with KQx, and that declarer is marked with most of the missing high cards, so some sort of trump promotion is the best chance for the defence.
