Recently I bought the new Raspberry Pi 4B with 4GB ram, freeing up my 3B+ to use as an Ender-3 server. Initial tests were impressive, the extra ram and faster processor minced the last generation in benchmarks. Able to support dual 4K displays (albeit from micro-HDMI), it could be used as the main PC for light tasks such as browsing. However, at idle it runs over 60 degrees. After looking online many others had even higher idle temperatures due to the lack of airflow when using the starter kit case. While this temperature is fine for testing, I wouldn't feel comfortable leaving it on as a server under load. After testing a 40mm fan in push and pull, it seemed that the push configuration was best, drastically bringing the temperature down to around 40 degrees.
I printed a case that protected the board while allowing it to breathe. With it being the first Pi to have true gigabit ethernet, I added an external hard drive to one of the USB 3 ports and have been using it as a NAS (Samba) for a few weeks. Taking it a step further I've turned it into a Plex Media Server, able to serve all the devices in our home with video. Although it doesn't have enough power to transcode anything but audio, most of my devices can playback the files directly. I've tested 5 clients direct playing at once without it breaking a sweat. Overall I'm happy with the setup, although the fan is a little loud. I'm considering adding a 40mm Noctua, in the meantime, I've bought heatsinks to see if it can go fan-less.