If the firmware works I wouldn't expect there to be any major problems as this update is only a couple of versions newer than your current firmware.
If your NVR is from the AUS region and this firmware is just completely incompatible, the update will just fail and not complete.
If the upgrade is successful, you probably won't be able to downgrade to your old V3 firmware as the upgrade will have stepped you into the V4 firmware versions which bring lots of new updates including the new V4.0 GUI.