When I booted Manjaro on my RPi 4 it could see networks, but could not connect, including not to a 2.4 GHz open (no security) network.
I did the usual user name set up etc. and rebooted.
Then I opened a terminal and ran (I am in the US)
sudo iw reg set US
This allowed me to connect to my usual WPA2 network.
Then I installed CRDA
sudo pacman -S crda
I rebooted and then edited /etc/conf.d/wireless-regdom
sudo nano /etc/conf.d/wireless-regdom
and deleted the # in front on my region setting line (US).
I rebooted and the network connected.
I hope this helps someone. I hope this is made part of set-up at some stage.
The probable reason this issue is not universal is that the region only matters for certain regulated wifi channels (they exist on both 5GHz and 2.4GHz), so if your router is not using those your Pi will connect.
I am a brand noob user so if this is all misleading please take it down. It worked for me.