A Prototype

10 7 5
A K 10 8 3
10 4
K Q 9
A J 3 2 9 8 6
J 5 4 2 Q
9 2 J 8 6 5 3
J 10 6 7 5 3 2
K Q 4
9 7 6
A K Q 7
A 8 4

EastSouthWestNorth
Pass 1 Pass 1
Pass 2 NT Pass 3 NT
All pass

Whenever I see a plus 4 over a bid, I hafta see if there has been grievous underbidding or poor defense, or just a lot of lucky placement of the cards. Well, actually, with 30 hcp's, little slam couldn't be stopped owing to a rather lucky break in hearts and that ol' Law of Restricted Choice. A spade can be generated, giving declarer 5 hearts, a spade, 3 diamonds and 3 clubs. But this declarer made seven! Now, how'd he do that? Given the placement here, you can guess that it was a poor handling of a four-card suit. And it shouldn't be difficult to figure out which one.
I won't go through the whole play. Suffice it to say that after a low spade opening lead and 7 more tricks, declarer had one spade, 5 hearts, two clubs, and one more club winner in dummy, while East held four diamonds and the 9 of spades. And guess what he threw on the queen of clubs!
There really isn't much excuse for that. East has about 3 reasons to hang onto his four-card diamond suit. One is that he's only got the 9 of spades when there are four spades higher than the nine out, one of them showing in dummy! Why on earth does he want to save that card? So as to lead it to his partner? Well, you've got to get the lead first, and the chances of getting the lead by throwing a small diamond are drastically reduced. The second is that South bid diamonds. True, I've said elsewhere that a minor suit opening cannot be regarded as promising at least four cards. But it's better than nothing to bear that bid in mind.
And the third reason? Well, it's simply the principle that I've been expounding here that your four-card suits can be very valuable and that careless handling of them is probably the most common defensive error. Please remember also that you're probably the only defender who can protect the fourth round of a suit, and though it's not impossible that you both can, if dummy shows two cards in the suit then it's impossible for your partner to protect the fourth round if declarer and you both have four cards in it. I think I might term it a "four-card consciousness". You've just gotta know as soon as the contract is decided that though the fourth round of your four-card suit may not become important on the hand, it is a hundred times likelier to be worth a trick than a three-card suit headed by the 9! And a fortiori when your 9 is blank and you can see the 10 blank in dummy! . . . you can't win a trick with the 9 under those circumstances!
So, I offer this as a prototype for all four-card suits (okay, four cards or longer, as this suit originally was). Take care of those four-card suits, and they'll take care of you.