After doing some research I found that the 4g symbol will stay lite as long as you have 4g service. The arrows below the symbol let's you know if data is being sent or received. If your location setting is set at a high accuracy then you will use both Wi-Fi and data. If messages are being sent or received then data is being used. There are several settings and functions of your phone that require mobile data even if you're connected to Wi-Fi. I wouldn't worry about it unless your seeing a dramatic increase in data usage.