The NFL officially announced the suspension later Friday.
Turbin announced the discipline on his Twitter account and acknowledged that he made a mistake.
“Today it will be made public, that a urine sample I provided to the NFL has resulted in a positive test of a banned substance under league policy with regards to Performance Enhancing Substances,” Turbin wrote. “It absolutely kills me that I allowed this to happen. It has always been a priority in my life to display a high level of leadership both on and off the field. I strive to be a person that people can look up to and strive to exemplify the definition of hard work.
“Unfortunately, I have made a mistake. A mistake that will not only have personal consequences, but will affect the Colts organization, an organization that I am extremely appreciative to be a part of. For that, I am truly sorry.”
The four-game suspension leaves Turbin eligible to return to the organization on Oct. 1, one day after a Week 4 contest against the Houston Texans. He will be able to play in an Oct. 4 game against the New England Patriots.
Turbin, 28, had just 53 yards on 23 carries in six games last season before being placed on injured reserve with an elbow injury.
He has rushed for 1,344 yards and nine touchdowns in six NFL seasons with the Seattle Seahawks, Cleveland Browns, Dallas Cowboys and Colts.
–Field Level Media