Work Out featured which army ranger and fitness model in its fourth season?
Work Out:  The fourth season, which revamped the show's concept, mainly featured top fitness model and former army ranger Greg Plitt, who was the only cast member to return from the previous season. Greg Plitt: George Gregory Plitt, Jr. (November 3, 1977 – January 17, 2015) was an American fitness model, actor and former Army Ranger.
George Gregory Plitt, Jr.