Day 4: 12:00 A.M.-1:00 A.M. - S4-E18
Continuity mistake: To get crucial information from Prado, Jack offers his resignation to Buchanan, and takes Prado on as a private citizen. But Jack doesn't work for CTU; he works for DOD, and is at CTU on a provisional basis.
Chosen answer: Nobody knew, not even the writers, until at least halfway through the season when they were writing the end. They wanted to keep all options open.
Nick N.