in 4 seasons of competition.

Student enrolls in the fall, which starts his 5 year clock running. He doesn't play football, so he doesn't use up one of his 4 seasons of eligibility, but the 5 year clock is running, even when he returns home to work and prepare for the mission. Once the mission starts, everything pauses. At that point he has 4 years to compete in 4 playing seasons. So coming off his mission, he'll be a redshirt freshman who can play for 4 years.