Fourth Minor League Option
Most players get only three minor league options, but a player who has accrued less than five "full seasons" is eligible for a 4th minor league option.
For the purpose of determining eligibility for a 4th minor league option, a player accrues a "full season" when he spends at least 90 total days on the Active List of an MLB and/or minor league club or clubs in a given season, or at least 60 days but less than 90 days (total) on the Active List of an MLB and/or minor league club or clubs, followed by a Disabled List assignment where the combined time spent on the Active List and Disabled List equals at least 90 days.
Optional Assignment Waivers are not required when a player is sent to the minors using a 4th minor league option. and if a player qualifies for a 4th minor league option year, the 4th option is available for that entire MLB regular season.