How do I unlock Barnet's briefcase?

1. There's a four-digit code.

2. Remember that deck of cards?

3. There's a clue hiding in there.

4. Notice how some cards are missing? Well, there are.

5. There are five cards missing. But we know where one of them is.

6. One of the missing cards is the ace, and it's up Barnet's sleeve.

7. The rest must be a clue.

(SOLUTION FOLLOWS)

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7. The cards missing are the 4 of clubs, the 8 of diamonds, the 2 of hearts and the 5 of spades.

8. Enter these numbers in alphabetical order of suit to open the briefcase. 4-8-2-5.