...all four are pretty dependant on each other.
If you wish to assign an order of inportance, it would have to be based on individual needs at any given time.
I say that posture, including stance, is what is most important for a begginer to learn. Then movement, footwork, would be next.
Of course you have to be relaxed to be able to do anything, to be aware of things around you, and to be able to react if needed.
Yet, you are can never truly be relaxed without good, controled breathing.
I have believed for a long time, that if you control your breathing, you are pretty much in control of your self. Physically & mentally.
Heck, the only people who don't breath, are dead. lol