I done this many times on long layovers because I love to walk and I like checking out the Intl Airplanes.
20 minutes sounds like a fair average.
Note that from T4 to TBIT it isn't a tunnel ... at least not in the sense of being underground. It's actually one level up from "gate level" with a glass wall overlooking the ramp. As you enter T4 from the T6-T5-T4 tunnel, turn right as if you want to exit T4. Just before getting to the end of the concourse (where passengers just finished with TSA processing are entering the concourse) turn left. The escalator up is a short distance. There are a few turns as you join TBIT, but just keep following the hallway, it will drop you into the main lobby .... all airside.