Temperature graph

Here you can see three different cooling scenarios under the same conditions. Ambient temperature was stable at 23.3˚C , no overclocking or overvolting was applied. No video output.

RPI 4B no case shows thermal throttling with 3 cores loaded at 82˚C and definitely a demise of frequency ramp with all 4 cores loaded, peaking temperature at 85˚C.

Cooling with CooliPi 4B passively shows top temperature at about 46˚C

Adding a Noctua NF-A6x25 5V PWM fan running at 3.3V shows further decrease of temperatures. Top temperature was around 31˚C.

The temperature delta between the BCM chip and a heatsink surface is about 9˚C High quality thermal grease between the contacting places reduce the delta by about 4-15 ˚C compared to no-grease case, depending on the pressure between the Raspberry Pi and a heatsink. The 9˚C delta shown is with a MX-2 thermal grease under video transcoding load.

You can see more on an overclocking page.