Please see the following message from DJI regarding compatibility issues between DJI GO 4 and the newest iOS mobile release:
Please note that there are some compatibility issues between the newly-released iOS 11.2.5 and our DJI GO 3.1.24, 3.1.26 or DJI GO 4 4.2.4 apps. The drone platforms potentially affected by this update are the Phantom 3 series, Phantom 4 series with HDMI module and the Inspire 1 series. The compatibility issue may disrupt the ability of the remote controllers of these platforms to connect to the DJI apps. Our developers are working on solving this issue as soon as possible and will notify you once we have released a fix.
To minimize impact to your user experience and flight stability, please refrain from updating your iOS to the 11.2.5 version until further notice. If you have already updated to the latest iOS 11.2.5 version and want to fly your drone as we work on resolving this issue, you can choose to roll back to the previous iOS 11.2.2.
We apologize for any inconveniences this may cause and will keep you posted on our progress.
DJI Product Technical Support Team
How do I know if I have updated to 11.2.5?
Go to settings>General>Software Update
How do I know my current software version?
Go to settings>General>About and look under Version
What should I do?
Of course, please always follow your company's policies regarding OS updates as well as the manufacturer's guidelines.
If you have not updated to iOS 11.2.5:
Consider not updating to the version until DJI releases a patch for this issue in the newest DJI Go version
If you have iOS 11.2.5:
Please keep your aircraft grounded if it falls under the conditions listed from DJI until the issue is handled. Or, you can consider rolling back to iOS version 11.2.2.
*Please note that iOS rollbacks are not supported by Apple, and should be done only at your own risk. You should back up all data before you begin, and follow all of your individual company policies. When in doubt, always follow the manufacturer's guidelines.