Actually, in his writings, Beck suggests 0.67 Hz to 3.9 Hz. Later the 3.9 Hz was just rounded to 4 Hz because the circuit was not that accurate anyhow.
>- Some people erroneously say that Beck chose 4Hz because it was a harmonic of the Schumann frequency.
Actually, 3.915 Hz is a subharmonic of the Schumann frequency ( 7.83 Hz), Which means that the Schumann frequency is the second harmonic ( approximately ) of the Beck Blood Electrifier. This frequency would therefore be completely absent from the signal.
A signal of 2.61 Hz would generate 7.83 Hz as the third harmonic.
>- Electric current, like the current of a stream or river, seeks the path of least resistance from point A to B. So if the blood in veins and arteries is more conductive (ie: offering less resistance to electric current flow) then current will tend to take that route instead of rougher going in lymph muscles tendons bones etc.