I use TiVo Stream 4ks and have USB storage installed and configured as device storage. Will the channels app on the Tivo Stream 4K use the USB storage as the buffer for live TV pause?
Only if you have moved the Channels app to the USB drive. Otherwise it will use the main device's storage.
(However, in my personal experience this has led to some instabilities and caused the app to crash at times. I know others have had good success, though, so YMMV.)