Can't speak for the Army, but in the Navy it was virtually unheard of to promote BTZ. About 10 years ago there was ~5 guys that did select BTZ to O-4 and they all struggled and most failed to make the subsequent rank. I think there was 1 dude last year that was BTZ to O-4, so it's rare.
Making O-4 is the goal simply to be able to retire, the Navy has for years let O-4s get to 20.
For comparison, last year the Navy promoted ITZ at about 70% to O-4 and about 60% to O-5.
My understanding of the merit based reordering is that the decision on who goes where was made at the board. If Generals are bickering now about who goes where that's just absurd. Not saying it's not happening, it just defeats the intent of the FY-18 NDAA that authorized it.
*Edit to add that my info is for pilots.