I think these are the possible scenarios for a player exhausting his eligibility.
1. Player plays first four seasons. Total of four years in school, four full seasons played
2. Player plays four seasons and takes one redshirt year (usually freshman). Total of five years in school and four seasons played.
3. Player takes a redshirt in a season and in a subsequent season gets seriously injured early enough in the season to qualify for medical redshirt. He gets the medical redshirt. Thus, six years in school, four full seasons played, one partial season played, one full season out.
4. Player does not take a redshirt but is injured in a season early enough to qualify for medical redshirt. He is allowed to count that season as his redshirt season even though he had played in a game. He then plays out his eligibility but cannot take a voluntary redshirt at any time. Total of five years in school, four full seasons played, one partial season played.
5. Same as above except player is injured in a season after his medical redshirt year but still would otherwise qualify. I think he can still take the medical redshirt if granted. Thus, six seasons in school, four full seasons played, two partial seasons played.
I think the jizz is that you can get up to two seasons of eligibility back. Either they redshirt and then are eligible for one medical redshirt or if they use the medical redshirt first they can only then get another medical redshirt if the circumstances arise.