I'm not experiencing this. Priority 4, 5 and 6 are distinct and operate as expected, they are not performing as a single priority (4).
I'm using Bento Buddy to export my animations, with the above priorities.
If I play a priority 4 and then a 5, the 5 does play.
If I play a priority 5 and then a 4 the 4 does not play.
The same combination with 6 and the others.
Using turn, walk, fly etc. do not appear to interfere with my above priorities.
Maybe I misunderstood what the issue is, I wouldn't mind being enlightened.