Spongebob, agree with everything and want to just add to number 3.

Getting to 18 years as an O-4 buys you 20 according to Title 10. However, in the past, that was when guys were usually getting looked at for the second time for O-5. These days guys are being looked at for O-5 at the 15-16 year mark, which means they could see the second pass as early as the 16-17 year mark.

So in the past it was coincidence that the board results matched up with title 10 and O-4 got you retirement, these days it isn't always the case. Guys may have been burned, but according to Title 10, the only guarantee for retirement is hitting the 18 year mark, not the promotion to O-4.

Right now, at least in the Marine Corps, consider it a "gentleman's agreement" that O-4's are allowed to stay for 20. It can change instantly at the whim of the Commandant, just like changing from green t-shrits to tan and back to green.