It is not stated anywhere. But how realistic is to keep all your devices out of date?I am still waiting for someone to show me where it says this update is required. No one that is convinced its forced can show me where it states this. If DJI can update a piece of software on a device that is not connected to the internet that would be nothing short of black magic. So while you voluntarily update I will continue to fly mine just fine the way it is.
You could even buy a new iPhone running a next version of iOS and DJI GO 4 current version could have incompatibilities with iOS next version. Examples are many and have been mentioned before. If you don't want to be convinced, no one can push you towards any direction. It's just not realistic in 2017 to stay out of date.