Ubuntu is a much more stable and mature 64-bit distro to use Pi 4 as a small server. So here is how you can fast setup the cooling fan and add a shutdown and poweron button.
By the time of writting, I was working with Ubuntu 22.04.2 LTS (jammy) in a Raspberry Pi 4 Model B 2019 Quad Core 64 Bit WiFi Bluetooth (4GB).
sudo vi /boot/firmware/config.txt
Comment the line bellow to disable I2C. If you need I2C, read on and change the default fan pin to another one not used by I2C.
#dtparam=i2c_arm=on
Add this lines at the end of the file:
# Custom config
dtoverlay=gpio-shutdown,gpio_pin=3
dtoverlay=gpio-fan,gpiopin=12
Done. GPIO PIN 3 is now your shutdown and power on pin. Wire it to any GND pin and you Pi will shutdown in on and power on if off. A button may be handful.
See the Pi's GPIO layout for reference.
The /boot/firmware/config.txt file we just edited is like a BIOS config for the Pi and Ubuntu fully supports it. This Pi's documentation has all the details about it and what you can do with it.
To know all overlays available to you, just take a look at this file in you system:
less /boot/firmware/overlays/README
To know more about Pi's Overlays, here is the docs.
You can find more tips on how to change your Pi's Ubuntu 64 config in this Stephane Chauveau's GIST, that was my starting point.