The fourth season of the police procedural drama NCIS was originally broadcast between September 19, 2006, and May 22, 2007.

Special Agent Leroy Jethro Gibbs left NCIS at the end of season three after a terrorist attack had been successful because his superiors did not heed his warnings in time.

New characters introduced in this season are Michelle Lee, who was briefly on DiNozzo's team and was transferred to the legal department upon Gibbs' return, and (already in the final episodes of season 3) Gibbs' former boss and mentor Mike Franks, both as recurring characters.

Also, albeit later in the season, Army CID Lieutenant Colonel Hollis Mann is introduced as another love interest for Gibbs.

In December 2006, Bill Keveney from USA Today announced that the "CBS investigative drama NCIS topped the Nielsen viewership list for the first time with 17.4 million.