The speed of sound is 340.29 meters per second. That's 34029 cm/sec 34.029 cm/ms .034029 cm/us. Invert to get 29.387 microseconds per cm. Divide your round-trip microsecond value by 59 to get one-way cm.
My guess is that there is something wrong with the transmitter or receiver of that unit and it is not seeing the echo at all. The numbers you are getting correspond roughly with the stated 4-meter range of the sensor. I think the receiver is timing out at maximum range.