Beck said that Clark Zappers "dont work". I would add that Hulda was a complete nut job that endangered the lives of many cancer patients. My advice is to completely ignore everything the woman ever said. She generated interest in electrotherapy but introduced so many wrong concepts.
Why 4Hz? I suspect Beck started experimenting with direct current because the lab studies of electric current against HIV was with direct current. They only proposed alternating current in the subsequent patent without having tried it. I also experimented with direct current blood electrification and after 3 months my blood was completely wrecked and I was so sick I really thought I was dying. DC is great for treating localized bacterial and viral infections (herpes etc) and even fungal infections but is horrible for blood electrification. Some people erroneously say that Beck chose 4Hz because it was a harmonic of the Schumann frequency. Not true. Harmonics less than the original frequency are at 1/3, 1/5, 1/7 (of the original frequency) and on and on with each smaller frequency having significantly smaller power. 4Hz is 1/2 the Schumann frequency and so is not a harmonic of it. I think Beck tried different frequencies and just settled on that one because he liked it (for effectiveness and minimal problems other than causing blood cells to be more absorptive of chemicals).
About current distribution- Without any proof I still have a theory. I believe the blood has more conductivity (than lymph, flesh, or bone) and so when you electrify from wrist to wrist or ankle to ankle the electric current eventually finds most of itself in the main blood arteries (in the groin area and the arteries between the shoulders) which is where it is concentrated enough to have maximum effects against microbes. 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.