Check your apps. Some apps are not optimized very well. Web browsers seem to eat power at a much faster rate. Also, if you don't need your location/bluetooth or sync I would turn those off. Location will really destroy the power. I turned off those three services. Only turn them on when needed. Made a pretty big difference on my G4's. Also I'm sure you already know how screen brightness can hit the battery as well. I usually run mine at about 45% while indoors. I will only turn it up to 100 percent while outdoors. Last thing I would pick up another battery on Amazon. They are dirt cheap. Make sure you buy the oem one made by LG and it generally comes with the white LG charger. I've seen two kinds of LG batteries for the G4. Some are rated at 2900 mAh and others are 3000 mAh. Obviously you want to secure the larger capacity. Avoid buying non LG batteries. I did and that one definitively has shorter life even those it rated at 3000 mAh. Being that you have a referb I would get another battery. I did the above steps and have pretty decent battery life on my G4's. My standby time is also really good. Sometimes it will still show 100 percent in the morning. Also worth mentioning is my G4's are unlocked and get their updates directly from LG. No carrier bloatware or code that can mess up the base OS.
Now for the bad news. You got a referb. In case you did not know G4's shipped with a major defect for the first 5 months of production (defect affects all regions globally
. Apparently a loose contact on the main board. Eventually the defect will show its ugly head and bam, your device will be bricked into an endless boot loop. What you need to do is look at your box for the serial number or better yet on the device itself. Your box may not be the original. Under the battery you will see the first three numbers of the serial number. If you see something starting around 505 to 509 then your device is one of the affected devices. There would be no way to confirm if LG swapped out your main board. If you have one of the earlier build dates I would seriously try to return the phone.