The software version 5.x is a major release and cannot be downgraded to software version 4.x the normal way.
You will receive an "error" saying it is not possible.
If you need to downgrade, it must be done using the FailSafe mode.
To enter the FailSafe mode you must enter "/failsafe" in the URL after the IP-address.
Example: http://192.168.0.100/failsafe
Enter the ADMIN menu -> System maintanance:
Change Firmware.
Upload the 4.x software, Select the 4.x software, Set active and "replace all"
That's it.