Please update the USBTether software to Version 1.6 or higher.
Then please try the following.
Important: Make sure you are using latest version of your cameras firmware and USB Tether.
1. Put SD card in GH4/GX8.
2. Put camera in PTP Mode.
3. Power off camera.
4. Attach camera via USB cable diectly to computer. Do not use hub.
5. Start USBTether.
6. Power on camera.
If this doesn't work repeat and swap steps 5 and 6.