This isn’t a help request, but a quick how-to on how to decrease the minimum screen brightness, since the default minimum brightness is far too high.
First of all, you will need root. I will not tell you here how to do it, google it yourself 
Also, be warned: you can mess stuff up using root. If you do, you’ll need to fix it yourself. If you don’t know what you are doing, instead forward this post to the Fairphone support and tell them to please incorporate it into their stock image, and don’t attempt rooting your phone. You have been warned.
So, disclaimer out of the way.
On your rooted phone, download the Automate app by Llamalab (or any other way to start a shell script at every boot of the phone).
Open the app and create a new flow. Edit the flow and add a single “Shell superuser command” block. Connect the “Flow beginning” block to the “IN” point on the “Shell superuser block”. Also connect the "Shell superuser block"s “OK” point to it’s “IN” point.
Now tap the “Shell superuser block” to edit it. In the “Command line” field, put the following line:
while true; do grep -E '^3$' /sys/class/backlight/panel0-backlight/brightness && echo 1 > /sys/class/backlight/panel0-backlight/brightness;sleep 1;done
Press “Save”, than the back button and then start.
Now every time the screen is dimmed to its lowest brightness, it will be dimmed a fair bit lower than that.
If you have verified, that it works fine, go to the settings of the Automate app, and enable “Run on system startup”.
How this works
The brightness of the screen is stored in the file /sys/class/backlight/panel0-backlight/brightness. The lowest value that the phone usually puts into this file is 3, but it can go as low as 1. So the script checks once a second if there is a 3 in the file, and if there is, it replaces it with a 1.
Warning: Don’t ever put it to 0, since that disables the screen backlight, and you won’t be able to see anything on your screen. If that happens, you will have to blindly pull down the notification bar and pull the brighness slider to the right, so that the screen backlight comes back on. Putting a bright light next to the screen might help you to see a faint shadow of the image on the screen.
This solution works on the FP4 @ Android 11. Other phones/versions might work slightly differently (for example, on my last phone the file was called /sys/class/backlight/backlight/brightness and the default minimum value was different, but it’s always below 10).