The DIY Life blog writes a tutorial in an extreme cooling solution for a Raspberry Pi 4B:

Every time I’ve made a new case for my Raspberry Pi, there are always a few comments suggesting adding another fan or making improvements to the cooling, so today I’m going to put these suggestions to the test by building a case that has as many fans as possible to find out if more fans really result in lower CPU temperatures.

I’m going to be comparing three configurations;

• A baseline small heatsink placed onto a Pi in my standard desktop case design with a fan on the side panel,
• An Ice Tower cooler on the Pi like I use in most of my designs,
• An Ice Tower cooler in a case where every possible space is filled with a fan.

I’m going to run each test at the Pi’s base frequency of 1.5Ghz and I’ll then overclock the Pi to 2.2Ghz and see how that affects the temperatures.

See the video below and more here.