Becks blood cleansing device is not a frequency device or a positive offset zapper. It kills with current instead. Ideally DC would be used but their is a problem with the electroplating effect. So Beck chose 4 Hz. The important thing is to keep the frequency low.
Their is a unit made by Scada Research that uses 2 slightly higher frequencies 4Hz, 16Hz, and 100Hz. This is to decrease the chance of electroporation.