Players that are held on MLB rosters at the beginning of the draft cannot be drafted in the rule 4. This means that nobody can cut a prospect that hasn't played an MLB game but is currently held on their MLB roster as an NA right before one of their rule 4 picks in order to "demote" said player to their MiLB roster. If a prospect that hasn't played an MLB game gets cut from an MLB roster before the rule 4 draft begins, then they can be drafted. In other words, a player can't gain MiLB draft eligibility during the draft.