Why use a heatsink and a case, when you can use a heatsink case? This black, anodised aluminium case for the Raspberry Pi 4 will give you 10-15°C of passive cooling under a full CPU load. This case is great for situations where you want completely silent cooling, like home media centers.
It comes with a thermal pad to provide thermal contact between the CPU and top case, and a handy Allen Key and set of hex bolts to attach the case together. The case gives you access to all of the ports, pins, and connectors.
More information is available on the website of Sparkfun.