I use 4.1.15 also. Sounds like you downloaded the apk without problem and you've got an installer. I didn't know which on of the three variations to get so I grabbed the one what had the most downloads. Installing the 4.1.15 apk should not be a problem if you:
- Uninstall the existing DJI GO 4 app whatever version it is otherwise the "old" version (4.1.15) won't install. I do a restart of my phone just to make things are clear.
- I used Astro file manager to install the "new" 4.1.15. No problems.
- Make sure the just uploaded 4.1.15 version of GO 4 is set so that it will not automatically update to a new version. If it updates, you get to start all over again (been there - done that). Go to the Play Store and turn off Automatic Updates.
I've had 4.1.15 for about three months now and I don't remember if you'll have to reenter the Spark and User information. No big deal if you do.
Good Luck and please let us know if you got it to work.