Four Card II

Q J 10 6 4
J 10 9 7 6
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J 8 6
8 A 9 2
Q 4 3 2 8 5
Q J 7 4 K 8 6 5 3 2
10 9 7 4 K 5
K 7 5 3 West North East South
A K 1
A 10 9 pass 1 pass 2
A Q 3 2 pass 4 all pass

Opening lead was the queen of diamonds, declarer sluffing a club from dummy and winning in the closed hand, East playing the 8. Declarer now led a trump to the queen and ace. East continued with a low diamond, the 10 being covered by the jack, dummy ruffing. On the second round of trump, West sluffs a heart, which declarer takes advantage of by cashing the A K, plays a low trump to dummy, ruffs a heart -- high, not for fear of overruff, but to allow a re-entry to dummy with a low spade. Declarer can now run two hearts because of that careless discard and take the club hook for 12 tricks.
West has three four-card suits here, which often makes a decision a little more difficult, but it shouldn't have here. By the time West has to make a discard, he has only low spots in diamonds, the K and 9 are still out -- and he got a strong signal from partner on his opening lead! So the low diamonds should certainly have been the first cards he parted with. He wasn't close to being squeezed.
There is no way for declarer to ruff out that heart suit to advantage and make 12 tricks on his own. The numbers might seem to add up -- let's see, cash the A K of hearts, get back to dummy and ruff a heart, back to dummy, ruff a heart, back to dummy to cash the fifth heart . . . Wait a minute! Where's he going to get all the entries he needs? His only entries to dummy are in trump, either by leading it or by ruffing a diamond. No, I think you'll see that declarer can't do all that on a 3-1 trump break. But better luck next time. West must absolutely hang onto that fourth heart when he can see 5 in dummy -- and at the very least, make declarer work for the queen if he does have the tickets to drop it.
Declarer does have the potential for eleven tricks on his own steam, thus: Second round of trump with the king (after ruffing that diamond at trick 3), A K of hearts, third round of trump to dummy. Declarer (with one trump left) now plays the jack of hearts to West's queen, discarding a club. He regains the lead with his last trump and can then cash two hearts and take the club hook for 11.
That's, incidentally, if declarer plays carefully. There are always declarers who think they should draw trump as soon as possible. We got nine trump, don't we? How can we run short? Well, he will run short if he doesn't unblock the heart suit and win the last round of trump in dummy. This isn't to suggest that you should count on declarer misplay, but that when you do your best, declarer misplay will often redound to a good score your way.
A club discard by West on the second round of trump from that four-card suit would allow declarer another path to eleven tricks, thus: Declarer takes the club hook (trick five after two diamond leads and two trump), cashes the ace and ruffs a club high, then draws the last trump with the king. The three of clubs is now the only one left. Declarer gets three club winners, a diamond, two hearts and again five spades (four in the long hand and a ruff of hearts in the closed hand eventually).

Declarer in fact has another path to 11 tricks (or again 12, if West is careless). Now the jack of clubs might be a valuable card. If the club hook loses, or if East has a king doubleton, that jack can be a winning card. Hence, ruff the opening diamond lead, take the club hook, which holds, cash the ace, intending to sluff a club on the ace of diamonds if the king doesn't fall. When it does, now a trump to the queen and ace. A diamond return? It doesn't matter. You ruff another diamond and draw trump with dummy's last two trump, cash the jack of clubs, and now you have the last trump and top hearts and ace of diamonds in the closed hand, losing at the end a club -- unless West was careless with you know what suit this time!
Which brings up the question, is West squeezed? He must keep four clubs (excepting for club leads) and four hearts (excepting for heart leads) up until declarer is locked into one hand or the other. Lemme see. No, he is not squeezed. Declarer doesn't have the communication to effect all the leads he wants.
In any event, you want to be very, very careful with your four-carders. Let your partner do some of the guarding when you've got to make a choice. Did he signal? Did he bid? Did he lead low, which traditionally suggests some length in the suit? And especially when you can see declarer has four cards (or more) in a suit do you want to hang onto it.