I think there are overall different issues in regards to 4th tier abilities. Obviously there would need to be other skills with fourth tiers because having one skill get that high makes the whole concept mute and wasted. Another issue (for me personally) is just that even if a skill were to get a fourth tier, it probably wouldn't be that useful. Nobody would have it because it is likely locked behind a five star unit, and some other abilities are just better. BA 4 would be pretty useless considering that most people prefer desperation or QP, and while guard is great, both of the aforementioned abilities are pretty much outclassed by "Armor Privileged" skills. Brash Assault is shut down by Bold Fighter, Vengeful Fighter for QP, and now Special Fighter for Guard.
I know that the other tier fours for BA, guard, and QP can be given to other move types, but people are more inclined to use armor cause it raises score. Maybe dragons could use them, but they have speed problems (outside of Summer Young Tiki) and are mostly looking for enemy phase. But armor still does that (IMO) better.