The LG G4 operating on T-Mobile US can now download a fix for the "Stagefright" exploit. The update is available both over-the-air and through LG Bridge, and pushes the software version from 10H to 10N.
In addition to patching "Stagefright", this update also adds a Slow Motion option for video, and separates notification sounds from the ringer sound in Settings. The update is pushing out now, but may take a day or two to appear on your phone. If you want to check manually, you can head into your phone's Settings and check for the update.
Thanks to AlwaysSmile on the forums!