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.
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Actually, the circuit uses low cost electromechanical relays that, relative to electronic devices such as transistors, are *very* slow. 4 Hz probably was the practical limit of whatever he used in the prototype.