You can select which network modes your mobile phone can use, eg. 3G or GSM. Your choice determines the data speeds you can achieve.
Tap the indicator next to "Enable 4G" to turn the function on or off.
4G/LTE enables you to achieve higher data speeds than when using 3G or GSM. If you turn on 4G/LTE, your mobile phone automatically switches between the network modes provided you're within range of a 4G/LTE network. If you turn off 4G/LTE, you can't achieve high data speeds.