If the third form build a shield, the fourth creates a whip, relentlessly punishing the opponent. The keyword is “attack”, from every direction, all the time, relentlessly. Physically, it is the most demanding form, because you have to stay in constant motion.
Again, the method of SaberAcademy Vienna is leaving the canonical description behind. Though acrobatics are allowed and encouraged in this form, nobody expects flying jumps and other wire-fu work. What it comes down to is advanced footwork, playing with measure and rhythm and the tactical element of “pressure”.
Though there are defensive techniques in this form, the preferred method of defence is constant offense. If the opponent has to concentrate on defence only, how could he launch an attack?