After the OS update from Android 5.1 to 6.0 (officially from LG) it is no longer possible to save Spotify downloads onto the SD card. Several ways to solve this problem in the Spotify support-forum do not work for the device LG G4.
Download quality set to "extreme"
Download a randomly chosen album
The download will always be saved in the following folder: internal\Android\data\com.spotify.music.
Reinstallation with addiditonally deletion of the above-mentioned folder on internal storage do not help.
Permission for Spotify to use the Storage was also manually set and did not help.
With the roll out of 4.7.0, this should now be fixed. You should be able to save offline music to SD Cards again by following the steps here.
We're also looking to improve the way this works in the future.
[UPDATE] With the roll out of 4.8.0, you can now select where to save your music: Go to Settings > Other > Storage and select where you want Spotify to store all your music. This option will be hidden if no SD Card is available or the SD Card is not accessible.