Wave 4 is a bearish reaction to a completed super strong rally in Wave 3 up. Every completed bullish A-B-C fractal gives birth to a corrective A-B-C move in opposite direction. In majority of cases Wave 4 down is unable to retrace more than 50% of the preceding rally in wave 3 up.
Wave 4 can be structured as any type of Corrective Waves.
After corrective wave 4 down is completed its lowest low marked by the red dot becomes a new main support level for the following corrective wave B of 5. If that new higher low established by the wave 4 gets violated by the next corrective wave we say that the five wave up structure gets invalidated because one of the formal rules is broken.