Fourth-Best Leads

Fourth-best leads are a standard defensive agreement on the opening lead and on subsequent leads. They are made from suits that are four cards or longer, and do not contain an honor sequence. Examples:

KT53 - lead the 3.
J9652 - lead the 5.
T7542 - lead the 4.

Examples of when NOT to use fourth-best leads:

AT53 - lead the ace against a suit contract. Against no-trumps, the proper lead would still be the 3.
KQ93 - lead the king against a suit contract (i.e. the top of any honor sequence). Against no-trumps, the proper lead would still be the 3.
KJT5 - lead the jack (i.e. the top card of an interior honor sequence) against both suit and no-trump contracts.
9872 - suit sequences headed by the 98 (or lower) require partnership discussion. Many players would lead the 9 instead of the 2.

Other Considerations

Fourth-best leads are used in conjunction with the Rule of 11. This mathematical formula is used by the opening leader's partner to count how many higher cards declarer has in the suit.