I just got the LG G4 phone, turned on developer options mode, tried running an app from Android Studio but it can\'t recognise my phone. I even tried switching from MTP to PTP b
Had the same problem and tried everything mentioned above with no success. What ended up working for me was downloading the LG Android USB device Drivers which can be found here: http://www.mylgphones.com/lg-android-usb-device-drivers
The latest drivers didn't work for me so I downloaded the second latest and voila! It worked! Hope this helps.