I'd guess that heatsink and a case with heatpipes would be sufficient for most, though perhaps not all, uses and silent, which for me is pretty important on a Raspberry Pi
The real downside is that mass plays a substantial role. Meaning thicker metal, larger cases, are better than thinner/smaller. Whereas a fan can accomplish similar results for less cost and space (but not passively).
If people don't think passive can work they need to check out Apple's recent Mac Pro that passively cools two Radeon Pro 580Xs. There's no limit, except cost.
Personally I still feel a combination of heat pipe, into heat sink, with a small progressive fan is the ultimate. If it is large enough the fan should stay off except under heavy load.
Just adding a small fan (trying to avoid blowing our own trumpet too much here) absolutely demolishes the cooling performance of any reasonably-sized heatsink. Presumably there's just not that much passive airflow over something an inch square, even with convection.
I still wonder what would happen if a full desktop-sized heatsink was coupled to the Pi 4's SOC.
I still wonder what would happen if a full desktop-sized heatsink was coupled to the Pi 4's SOC.
I've been tempted for a while to build a big case with some sort of mounting standard (maybe a Peek array?) so that I can add whatever hardware I want to it (mostly electronics). This seems like a job for that.
What I don't like about the article is that small fan have to spin fast (= noise) to generate enough flow. I'd much rather use a 80 mm fan, slowed way down, but I suppose I'd had to roll my own case for that. Better still, wait for someone to start offering this (or maybe this: http://blog.flirc.tv/index.php/2019/06/24/new-pi-4-cases/)
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.
Another approach is to just run the Pi submersed in mineral oil, with liquid convection then taking the heat away from the board and shedding it to the surrounding environment. We usually don't run our electronics like this (even though the general approach is widely used for cooling needs of all sorts) because mineral oil is gross and might even impact the endurance of our hardware - but the Pi is tiny and cheap enough to make this a non-issue.