Many of today's devices run on the 4G network. Unfortunately, 4G coverage is often spotty, and using it can quickly drain your phone's battery and put you over your monthly data allowance. You might want to use the 4G network only in a well-covered area, or switch to 3G when you're running low on power. Android makes it easy to do this. To switch networks, simply go toSettings > Mobile networks > Network mode and set your phone to CDMA only (3G) or LTE/CDMA (4G). These settings might vary depending on your location and which network you're on.
Source :- informationweek