V4 filters are differ from V3. You can't just put there shader and assume that texture coords are in [0,1] range.
I am sorry that you have to learn all of that, and I will make sure that the process will be easier for pixi-v5.
Filter Area
Thanks to @bQvle and @radixzz
First, lets work with the AREA. When you apply filter to container, PIXI calculates the bounding box for it. We are working with bounding box.
Invariant: maximal vTextureCoord multiplied by "filterArea.xy"