I have had the same issue with my Sprint LG-G4 I purchased a month ago. The phone would randomly shutdown and would only turn back on if I pulled the battery out and put it back in.
I did multiple factory resets and booted also booted into safemode and did not install any additional apps yet the issue continued.
I went to a Sprint store and they did a battery test and it came up fine. The phone actually shutdown while they were handling it so they were able to witness the issue (or the symptom at least). They said it must be an issue with the phone itself (i.e. manufacturer defect) but because I don't have insurance I have to pay $75 and they would order me a new one. I thought that it was ridiculous that I need to have insurance with Sprint to protect me from them selling me a defective product. The reps at the store were nice about it though but basically said nothing they can do without the charge unless I contact LG myself to make a warranty claim.
I called LG and their customer service was hard to deal with to say the least. Either way what they told me was that I would need to send the phone in to them and it would take at minimum 2-3 weeks until they could look at it and send back a new one if they see it as being defective. I told them I can't go that long without a phone but they said they cannot send me new one until I send the old one. The rep wouldn't give me any info about her or ID number for me to reference, so that was a little frustrating.
I realized after I hung up that I should not have to be dealing with LG since Sprint was really the one who sold me the phone and I am their customer and have been for years. I called up Sprint customer service and spoke to an extremely nice lady who heard out the whole situation and the issue about me having to pay $75 even though it seems they sold me a defective product. She put me on hold a bunch of times speaking to higher up reps etc. and finally told me that I should go back to the Sprint store and have them give me a new phone and she will credit what they charge me (she said it would be between $30 and $75) back on my account. She took down my number and said she is working tomorrow and asked what time I plan on going to the Sprint store. I made up with her that she would call me at 8pm as she said she got off at 8:45pm and I wasn't sure when I was going to get to the store.
I went back to the Sprint store today and told them the situation. They went back to look at the phone for a while with the techs and the guy came back and said that the tech said he has seen several people come in with this issue on the LG G4 and he feels that it is a software issue which LG is working on to fix and put out as an update. The guy then said that he can see on the screen the notes of my call with the rep last night and that they would credit my account for the cost. However, I need to pay out of pocket and they cannot bill my account (this was a little disappointing because I asked rep on phone about this since I share account with a few people and would be annoying to settle up for what I put out).
The other issue was that he said the LG G4 is not in stock there or in any area stores and is in fact on back order. But he processed the order and said it comes 1-2 business days but because of the back-order it could be longer. Either Way I still have my (defective) phone and can keep it until the other one comes in as he said it would come with a return slip to send back the broken phone. This is helpful since the phone does work some what and sometimes will behave and stay on with shutting down for a few hours.
I should be hearing from the Spring rep tonight to get the $75 credited and will hopefully receive a replacement phone soon in the mail.
I apologize for the long post but after reading what a lot of people wrote, I felt maybe my story could answer some questions as I learned a bunch of things through dealing with this issue that might be helpful to others and it (at least looks like) the issue should be resolved one way or another. I will try to update on what ends up happening.
I did multiple factory resets and booted also booted into safemode and did not install any additional apps yet the issue continued.
I went to a Sprint store and they did a battery test and it came up fine. The phone actually shutdown while they were handling it so they were able to witness the issue (or the symptom at least). They said it must be an issue with the phone itself (i.e. manufacturer defect) but because I don't have insurance I have to pay $75 and they would order me a new one. I thought that it was ridiculous that I need to have insurance with Sprint to protect me from them selling me a defective product. The reps at the store were nice about it though but basically said nothing they can do without the charge unless I contact LG myself to make a warranty claim.
I called LG and their customer service was hard to deal with to say the least. Either way what they told me was that I would need to send the phone in to them and it would take at minimum 2-3 weeks until they could look at it and send back a new one if they see it as being defective. I told them I can't go that long without a phone but they said they cannot send me new one until I send the old one. The rep wouldn't give me any info about her or ID number for me to reference, so that was a little frustrating.
I realized after I hung up that I should not have to be dealing with LG since Sprint was really the one who sold me the phone and I am their customer and have been for years. I called up Sprint customer service and spoke to an extremely nice lady who heard out the whole situation and the issue about me having to pay $75 even though it seems they sold me a defective product. She put me on hold a bunch of times speaking to higher up reps etc. and finally told me that I should go back to the Sprint store and have them give me a new phone and she will credit what they charge me (she said it would be between $30 and $75) back on my account. She took down my number and said she is working tomorrow and asked what time I plan on going to the Sprint store. I made up with her that she would call me at 8pm as she said she got off at 8:45pm and I wasn't sure when I was going to get to the store.
I went back to the Sprint store today and told them the situation. They went back to look at the phone for a while with the techs and the guy came back and said that the tech said he has seen several people come in with this issue on the LG G4 and he feels that it is a software issue which LG is working on to fix and put out as an update. The guy then said that he can see on the screen the notes of my call with the rep last night and that they would credit my account for the cost. However, I need to pay out of pocket and they cannot bill my account (this was a little disappointing because I asked rep on phone about this since I share account with a few people and would be annoying to settle up for what I put out).
The other issue was that he said the LG G4 is not in stock there or in any area stores and is in fact on back order. But he processed the order and said it comes 1-2 business days but because of the back-order it could be longer. Either Way I still have my (defective) phone and can keep it until the other one comes in as he said it would come with a return slip to send back the broken phone. This is helpful since the phone does work some what and sometimes will behave and stay on with shutting down for a few hours.
I should be hearing from the Spring rep tonight to get the $75 credited and will hopefully receive a replacement phone soon in the mail.
I apologize for the long post but after reading what a lot of people wrote, I felt maybe my story could answer some questions as I learned a bunch of things through dealing with this issue that might be helpful to others and it (at least looks like) the issue should be resolved one way or another. I will try to update on what ends up happening.
Mine eventually got stuck in the Accessibility menu and just kept looping through the 3 screens. I couldn't do anything with it, nothing worked to reboot it, and then it stopped taking a charge. I had to actually re-activate an old dumb phone that I had squirreled away in a cupboard, I learned fast how dependent I am on my little tricorder. 
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