please point out on the pimax website where it says 4k is 90hz?
the pimax 4k has 60hz (max) LED Lens.
the pimax BE 4k (business edition) has OLED lens but not same resolution as 4k and can go up to about 65-70hz
the pimax 8k is aiming for 90hz (currently in development) can only go 82hz stable
pimax uses 'brainwarp' technology, which effectively is showing a full image on the left eye, and blank on the right then showing the image on the right eye and blank on the left... it does this very fast so your brain thinks it is seeing both eyes at once, which reduces the stress on gpu/cpu load.
hope this clears it up?