convection cooling needs a decent gap between the fins for it to really work. This is why something like Noctua's passive cooler ( https://www.anandtech.com/sho

convection cooling needs a decent gap between the fins for it to really work. This is why something like Noctua's passive cooler ( https://www.anandtech.com/show/14486/noctua-shows-off-concept-fanless-cpu-cooler-up-to-120w-of-cooling-performance https://www.anandtech.com/show/14486/noctua-shows-off-concep... ) looks totally different from their tower coolers with a fan. Just slapping a normal CPU heatsink on the pi4 will probably work OK just do the thermal mass and exterior surface area of it, but you're probably not going to get much convection-cooling from it, either.