Since you went through the process of downgrading your S4 back to KitKat, try wiping the system cache partition. The system cache is different from the regular app caches and resides in its own separate partition than you where your files are stored so this won't affect your data. Power off your phone, then while holding down the Volume Up, the Home, and the Power buttons start it back up again. Once your S4 vibrates you can release the Power button but continue holding down the Volume Up and Home buttons. This will start your phone up into its Recovery Mode. Use the buttons indicated to navigate the text-only menus and select 'wipe cache partition'. This will only take a few seconds. Now select the option to reboot your phone.
As for Android System and OS being in the top three that's pretty normal. Also note the Display is always a big battery draw, when you wake your screen from its sleep mode there's power going to the back light, the graphics processor (GPU) is working, and whatever you're doing to interact with your phone is working the main processor itself (CPU). You might want to change the Brightness level of your screen in the Display menu in your Settings. The dimmer you set your screen the less energy it uses.