as an opening Modifier
In notrump auctions Modifier
If notrump has been agreed upon (or, often, was the last contract bid) then a jump to 4♣ is often Gerber, asking for aces.
Directly after a one-level suit opening Modifier
After partner opens in a suit other than clubs, 4♣ is a double jump and is therefore often used as a splinter bid, showing a singleton or void in clubs.