The benefit of a tetrapolar (4 electrode) measurement is that the electrode impedance is not included in the measurement. With dry electrodes, the electrode impedance can vary drastically with motion and varying conditions, so tetrapolar results in a much more accurate measurement, especially for wrist devices. With a bipolar (2 electrode) measurement, the measured impedance includes the electrodes, so this is only suitable for very stable electrodes such as wet electrodes or large-area dry electrodes (e.g. a chest strap).