“I don't get this heat sink (I could be the problem) it's way (way) too large for the cpu on th PI4. AND if you buy the official raspberryPI4 case fan it includes a neat little heat sink that does fit the CPU.”
Ahoy Jeremy! We use these oversized heatsinks with our Pibow 4 case ( https://shop.pimoroni.com/products/pibow-coupe-4 ) but we do sell smaller ones too if those would work better for your purposes: https://shop.pimoroni.com/products/heatsink
Feel free drop us a line at support@pimoroni.com if you need to return anything :)