At the moment my Setup runs on a PI Zero W with the ConBee 2 stick.
I was wondering if a simple SD swap (Zero to PI4) would be enough or if I have to setup up the PI4 with a new image.
The OS is not exaclty the same, I think it will work but you will need to update it, deconz use same SD card image for all hardware (but not for Pi 0) SD-cart images
Your problem is from deconz side or raspbian side ?
From deconz, no problem and can make a backup using phoscon.