I did a little test on this...I'm a truck driver so I get around to both 3G and 4G areas...and everywhere in between (this is what drains the battery).
The battery will last a good bit longer if the phone is switched to CDMA only...if you have a good CDMA (3G or 1xrtt) signal...the difference on my phone was measured in hours, not minutes.
I generally leave mine set to CDMA/LTE though, I'm always in the truck so I'm never far from a charger...and switching back and forth between 3G and 4G is a pretty big battery drain.
In a nutshell...Yes, switching it to CDMA only will extend the battery by quite a lot if you are in an area where the phone switches back and forth.
And even if you're in a solid signal area...it will still last longer in 3G.