Quick caveat... the four year athletic scholarship is only guaranteed at Power 5 conferences. A player cannot lose their scholarship due to lack of athletic performance but can for a variety of 'off the court' issues.Exactly, walkon's have no guarantee, the others are guaranteed four years of eligible college scholarship years.
Most other conferences only guarantee athletic scholarships for one year. In those conferences, a coach can choose to not renew a player's schollie simply because they are not performing on the playing field.