ND4 will cut the light getting through to your sensor by 75%
ND8 will only let 12.5% of the light get through.
These filters are useless for drone stills photography (unless you have a particular reason to want to force a slower shutter speed).
The CPF is a circular polarising filter.
It removes polarized light from the image, thus reducing reflections and glare, while at the same time increasing color saturation—especially that of a blue sky.
But a CPS is a pain to use properly on a drone because it has to be properly aligned with respect to teh sun angle.
You can set it up to work with the drone pointing in one direction but if you move the drone to face a different direction, you've now got a misaligned CPF which isn't going to do what you want it to.