I am suspecting that you used 4.00 Hz, which according to some sources is not the actual correct frequency.
$ Hz was the result of rounding up the actual frequency of 3.915 Hz which is 1/2 of the Schumann Resonance frequency of 7.83 Hz. That may be one reason that the work failed. If that is the case, frequency really does matter.
Also, You mentioned using DMMs as the basis of frequency. I have found that while possibly accurate for sine waves, many are thrown off by square waves. I only use precision frequency counters.