The lead of the fourth highest card in a suit. This lead is standard unless the suit contains no honour or contains an honour sequence. From Q-10-6-5-3, you lead the five; from K-9-7-6-3-2, you lead the six; from J-8-4-2, you lead the two.
By leading low, you keep your honours to capture those that declarer holds and as entries. By leading specifically fourth highest, you help partner gauge your holding in the suit.