Click Settings app.
Settings are displayed. Click Cellular.
Cellular settings are displayed. Click Voice & Data.
Voice & Data settings are displayed. As you can see, it shows a tick mark in front of 3G or 2g whichever you have enabled. Click LTE, which is another name for 4G.
The tick mark will now appear in front of LTE and the iPhone would now be ready to connect to 4G infrastructure of any carrier. So this is how we enable 4G on an iPhone running iOS 8.