4G only really kills battery when there is a lack of coverage. If you have great coverage, it adjusts accordingly. You should only drop to 3G if that coverage is better than or equal to that of your 4G coverage.
Yeah, what Bla1ze said. LTE usually kills battery for people because the networks around them aren't that strong. But if you have good coverage around you, I'd be more worried about data consumption more than battery consumption
BTW...do you know which Z30 sub-model you are using? And what carrier is it on that you're getting LTE?
(I was asking this question because I'm a huge fan of BlackBerry and love my Z30 but I feel I am not getting the battery performance as some seem to have. I use my Z30 quite a bit but hardly video...feel I still to plug in charger when I am in office...)