Amir Hekmati became the first American to receive the death penalty in Iran in more than 33 years in January 2012 for espionage, waging war against God and corrupting the earth, according to the Free Amir website set up by his supporters.
That death sentence was later overturned by a higher court, but he was subsequently convicted in a secret trial of "cooperating with hostile governments" and received a 10-year prison sentence, according to the website and a CNN interview with Hekmati's sister, Sarah, last year.
"Today marks four years that our brother, son and American Marine veteran, Amir Hekmati, has been imprisoned in Iran," Hekmati's family wrote in a statement.
Kerry called on the Iranian government to release Hekmati on humanitarian grounds.
"He traveled to Iran as a tourist to visit family, committed no crime, yet this nightmare continues," Kerry said in a press release. "He received approval for his trip from Iranian authorities and was reassured that he would not be punished for his service in the Marines. Now he has suffered in an Iranian prison longer than any American in history."
Hekmati arrived in Iran to visit his grandmother, the family's website says.
On Saturday, supporters of the Free Amir campaign commemorated the anniversary and protested his imprisonment with an event in the family's hometown of Bay City, Michigan.
There are currently four known Americans detained or missing in Iran: Hekmati, Jason Rezaian, Saeed Abedini and Robert Levinson.
This past Wednesday also marked the 400th day since Rezaian, a Washington Post correspondent based in Iran, was arrested in Tehran. He is currently awaiting a verdict in his closed-door trial in which he has been accused of espionage and other offenses, including "collaborating with a hostile government" and "propaganda against the establishment," according to the Post.