My RockPiX overheated easily when maxed out for a while(throttling at 75C and temp up to 80C). I suspected there was thermal resistance between the heatsink and the SoC. This because the contact area is bubbled and painted.
So I bought some sandpaper and polished the contact surface, and improved the temperatures hugely.
From needing a fan to being cool fanless.
In this video I show how I done this, and how much difference it made.
This might also work with the heatsink of the RockPi4. I don’t have that one anymore to try.