In 2004, Poppinga started every game of his senior season, shifting near the end of the season from defensive end to outside linebacker. He finished third on the team in tackles with 79 (35 solo), also accumulating six sacks and one forced fumble, and for the third time in his career was selected for first-team All-MWC honors. Poppinga was also a candidate among 58 others for the Bronko Nagurski Trophy for the nation's top defensive player. He was named defensive player of the game for a September 4 contest against Notre Dame in which he posted 12 tackles (seven solo), a sack, a forced fumble and a fumble recovery.