Since you’re on an iPhone 7, you have to hold down the power and volume down button for up to 10-15 seconds are the phone to do an emergency shutdown. If that doesn’t work, it could even be the battery in your phone getting worn out or may be in bad health. Also the screen resolution of iPhone 7 is not even FHD so that means you can’t really watch 4K videos at the resolution it is meant for. But 4.5 GB for a single video is definitely a lot for a phone to handle.
Try powering on yourJBL Charge 4 first and wait for the LED indicator to turn on. Then try holding down the play/pause button and the volume up button up to 20 seconds and that should factory reset your device. Then try again and see if it makes sound. I personally have a JBL Xtreme and I’ve had similar problem and factory reset fixed mine. Hopefully it works for you too.