Chris Castallo is poised to exit CBS after nearly four years as the network’s head of alternative programming.

Castallo, exec VP of alternative programming, will stay on until a successor is named. A CBS spokesman confirmed Castallo intends to leave when his contract expires in September.

It’s understood that Castallo and CBS Entertainment president Glenn Geller have been discussing a transition for some time. CBS’ search for a new alternative programming chief is complicated by the fact that Geller is on a medical leave of absence as he recovers from a mild heart attack.

Castallo’s departure from the Eye is described as an amicable parting. CBS has failed to launch any lasting unscripted franchises on Castallo’s watch as department head. Most recently, the manhunt competition “Hunted” was a disappointing performer for the network despite a big premiere send-off after the AFC Championship game in January.

At the same time, Castallo is credited with doing a good job of shepherding the longstanding pillars of CBS’ alternative slate — “Survivor,” “The Amazing Race” and “Big Brother” — with innovations that freshened the formats. He also supervised “Undercover Boss.”

Castallo joined CBS in February 2007 as senior VP of alternative programming. He was upped to department head in August 2013.

Before CBS, Castallo was senior VP of development at NBC, working on such series as “Heroes” and “Studio 60 on the Sunset Strip.” Earlier in his career, Castallo worked in development at Tollin-Robbins Productions, helping to steer such shows as the WB Network’s “Smallville” and “One Tree Hill.”