I’d like to attach a flash drive to transfer photos & videos to my desktop
To transfer pictures from your phone to your desktop just connect a micro usb cable from the phone to a USB port on the desktop. Also, if you are using the Photos app on your phone, you can transfer your photos from here:
OTG cables will work as you describe on any phone or tablet running a version of Android newer than 3.0 if the manufacturer supports it in the ROM they put on the device. I’m sure the G4 is a supported device. You can install an OTG checker app if you wish to verify this.