The FLY app also requires a much higher spec phone than the Go4 app. If you are thinking about buying a drone that uses the FLY app check your phone or tablet's compatibility before proceeding. One of the simplest checks is perhaps to download the app from DJI, try to install it and then see if it will run.
If the current version of the app does not run on your phone and your phone is Android then do a google for "DJI fly app apk" and you should find one or more repositories for them. Do a google for the earliest version of the app that is required for your prospective drone and download the relevant apk etc. etc.
