If you want to try it, it's via Settings, Tethering & Networks, Mobile Networks, Network Mode.
Also of course in Settings, Mobile Data, you can set a limit to your data use by setting the sliders. The yellow slider warns you when you reach it, and perhaps set a bit higher, at the red limit your data use will be cut off.
I can't see how data use via 3G rather than 4G would be less, as long as you're doing the same things, but I stand ready to be corrected. I should have thought you would simply do those things slower.
Posted via Android Central App