Season Four, also known as Day 4, of the television series 24 premiered on January 9, 2005 and aired its season finale on May 23, 2005. The Season Four storyline starts and ends at 7:00 am.

Season Four is set 18 months after Season Three. Jack is now working for Secretary of Defense James Heller after being fired by CTU due to his heroin addiction. As the day begins, he gets caught up in an elaborate terrorist plot which involves James Heller and his daughter Audrey Raines.
 
Unlike previous seasons, which focused on a singular threat, multiple enemies, and conspiracies, this season is based around one main enemy: a terrorist named Habib Marwan who controls a series of Middle Eastern terrorist cells that launch attacks against the United States. Rather than three large acts, this season is divided into several smaller acts depending on what terrorist threat Marwan came up with.
 1.The kidnapping and planned broadcast of the execution of the Secretary of Defense.
 2.Nuclear power plants across America are sent into a forced meltdown.
 3.Terrorists steal a stealth fighter to shoot down Air Force One.
 4.Jack and Marwan scramble to get ahold of the nuclear football.
 5.The terrorists steal and launch a nuclear missile.