In Stud, four cards of one suit, or four cards in a row.
One card away from a flush or a straight. Its value depends heavily on how many of the cards you need already show in opponents' face-up cards (those cards can no longer come: they are dead). Example. With four hearts on 5th street, 9 hearts remain. If 3 of them show in other players' upcards, only 6 hearts can still come. In Stud you count the upcards and decide whether to continue from how many cards you can still draw.