I was also a passed-over O-4, and was forced to retire exactly at 20 years. We also had another guy in my unit who was in the same situation, passed over but with an ADSC showing beyond his 20 yr point. He thought that meant he would be allowed to stay past 20 to fulfill his ADSC, and was caught unawares when he got a RIP notifying him of his impending involuntary retirement.

The rules as written are that Majors can't serve past 20, even with an existing ADSC. But they are recalling retirees, so rumor has it that they're going to change the rules on mandatory retirement for O-4's to allow them to stay to 24 years. It'll all depend on how they word the change. Will it be completely voluntary to stay past 20? Will they make people stay to the end of their ADSC's, then it becomes voluntary to stay to 24? I'd expect the change to happen fairly soon, then you'll have a better answer.