Around the NFL confirmed the punishment Friday.
Pennel joined the Packers after going undrafted in 2014. He played a limited role as a rookie, tallying just eight combined tackles in 13 games. His defensive involvement increased during his second season in Green Bay as he recorded 25 tackles while appearing in all 16 games.
The 24-year-old Colorado State-Pueblo product has one year left on his contract. He's scheduled to make $600,000 before becoming a restricted free agent at the end of the season, according to Spotrac.
Weston Hodkiewicz of USA Today noted the team declined to comment on the matter for confidentiality reasons. Hodkiewicz also noted Pennel would still be eligible to take part in all offseason activities and preseason games despite the suspension.
The four-game suspension would take effect at the start of the regular season.
His absence means the Packers will likely need to add some depth to the defensive line during the offseason to fill the void.
It's a major setback for Pennel, who had started to carve out a more important role in Green Bay.