As I have z-wave, zigbee and a third stick for Schellenberg roller shutter, I could not manage all three in one raspi. So z-wave and Schellenberg are linked via SYMLINKS in raspi 1 (etc/udev/rules.d/99-com.rules). zigbee is on raspi 2, the only device at the usb port. Just place --dev=/dev/ttyACM0 in deconz.service and no trouble any more.
Actually my Deconz is on /dev/ttyACM1 and I added the option in the command-line as you mentioned but it did not work until I did both: adding it to the command line AND patching the /usr/lib/udev/rules.d 
where the latter is dangerous whenever there is a debian update.
Hi @JensD
thanks for your conclusions 
I share your pain - because I was too lazy to document too…
My short term solution is: I boght a second Raspi - one continues to host 3.4.4. and the second one is the sandbox for 4.0.0.
So I can look up all the detaills necessary “live”…