4♣ as an ask for aces. It consists in bidding 4♣ after non-forcing partner's bids 1/2/3 NT and unspecified trumps. 

Responses (modern):

• 4♦ - 0 or 4 aces;

• 4♥ - 1 or 5 aces;

• 4♠ - 2 aces;

• 4NT - 3 aces.

After response the next free bid (leaving 4NT) is an ask for kings (never 5NT).