Steelers general manager Kevin Colbert said in a statement that the organization was "disappointed" and that Gilbert will not appeal his suspension.
Gilbert will not be permitted inside the Steelers' facilities until after the suspension is served. He will be eligible to return to the team on Dec. 18, a day after the Steelers' anticipated clash with the New England Patriots at Heinz Field.
Gilbert issued a statement via his official Twitter account.
"I apologize to my teammates, coaches and the entire steelers family," Gilbert wrote. "Regretfully I inadvertently took a banned substance. I promise to come back in great shape and will be ready to play when my suspension is over."
The 29-year-old Gilbert has missed three games this season because of a lingering hamstring issue. He is in the third year of a five-year, $30 million contract he signed in 2015, and Gilbert's agent, Drew Rosenhaus, met with the Steelers last offseason about a potential contract extension down the road. His base salary for 2017 is $4 million, meaning he could miss up to $1 million during the four-game ban.
Though Chris Hubbard acquitted himself well in relief of Gilbert, the Steelers will be without a player offensive coordinator Todd Haley has called the best right tackle in the game. Many of the Steelers' designed runs go to the right side behind guard David DeCastro and Gilbert, who's played a big role in quarterback Ben Roethlisberger's declining sack numbers in recent years.
Still, head coach Mike Tomlin recently used Hubbard as an example of his expectation to see no dropoff when subs play significant snaps.
"Marcus Gilbert missed a significant amount of time. Chris Hubbard's play made it a nondiscussion," Tomlin said. "We didn't talk about it a bunch because his play was above the line. ... You guys know our mentality. The standard is the standard."
For insurance, the Steelers have second-year tackle Jerald Hawkins and have added tackle Jake Rodgers to their practice squad.
Gilbert is the third Steeler in as many years to miss multiple games because of suspension, joining Martavis Bryant (four games in 2015, all of 2016) and Le'Veon Bell (two games in 2015, three in 2016).