When battery saver turns on [at 20%] it shuts off WiFi when 4G is not in use but if I launch an app that used the Internet, WiFi stays disconnected and uses my LIMITED data.
I'm using an LG G3. The batter saver built into the phone. Once it drops to 20% battery the WiFi turns off. I shouldn't have to turn it back on manually when it says "only when 4G is not in use".
Just checked mine, the option is 'Turn off wifi if data is not in use'. Perhaps it's not referring to mobile data use, but to whether you're sending or receiving data via wifi at the time the battery saver activates.
If the feature is only referring to the instant battery saver turns on then that is a useless feature that has been draining my data and costing me money. Not happy.