I have watched two different video reviews about the Pi4. Both mention that it has a new, faster CPU, and that as the temperature of the CPU goes up, it will throttle the frequency back and run slower. So, while it is possible to run the Pi4 without a heat sink or active cooling with a fan, the more you ask it to do, the slower it will run. Basically, the CPU temps go up as the CPU runs faster.
It has been a while, but I remember running those tests on a Pi 3 with just a small aluminum heat sink, which made a very noticeable drop in CPU temperature both at idle, and heavy load, compared to a Pi3 without the heat sink. I also ran tests on a Pi3 with a heat sink with small cooling fan. The fan made the Pi3 even cooler, but I could not stand the "whine" when the fan was running.
For testing plugins I maintain, to make sure they work on old versions of Moodle, I use an Odroid XU-4 with Moodle 1.9.19 all the way through Moodle 3.3. It has an active cooling fan. The fan whine bugs me so much that I turn the thing off except when I am actively testing plugins for a new release.