If you mean the '4' button from Hijack, then it's just a "one touch" access for the "countdown timer" functions found on the main Hijack menu (press and hold knob until menu appears, then scroll to the "countdown timer" menu entries).

Basically these are used to automatically turn the player on or off, or to trigger an alarm ("beep!") -- can be used as a one-shot alarm clock, or sleep timer.

Cheers