Pokemon go keeps restarting
Maybe uninstalling/reinstalling isn't correctly deleting all the cache files - go into app settings, find Pokemon Go (while it is installed) and force close it. Then clear its cache and data, and try again.
If you are installing from some APK website (if the game isn't officially available in your area yet), perhaps try downloading another copy of the game from somewhere else.
EDIT: Also, perhaps just make sure your S4 isn't currently running PokeGo while you try to log in on your S7. There could be some kind of protection against that.