All the IOS Apps are saved in:
Home folder/Music/iTunes/Mobile Applications/
If you didn’t sync your iPhone, the old app should be there. Should look like this :
DJI GO 4 4.0.2.ipa (or older version)
If you did a sync and have a TimeMachine backup you can get it from older backup. and put it back in the folder /Mobile Applications/ (before erase the new one)
Then you erase the app (DJI GO 4) on the phone (not connected to the computer), and then connect and reinstall it from iTunes
It did the trick for me
If you never did a sync or have no TimeMachine and did a sync, there is no way to downgrade.