Well, there's the problem, isn't it. The schematic appears to indicate 1:5 stepup, but
if it really means stepdown, and you wire it backwards, then .... zappp.
100V is a lot, and 4V isn't very much. Who knows what's supposed to work? This is the
problem with ckts you find on the internet. You're the one being the real guinea pig,
and the guy who wrote the page may be a complete idiot.
The other thing is, piezo transducers are sound generators/vibrators in their own right,
so maybe this thing has nothing to do with "neurophone", and is just a vibrator on the
skin. So, be careful, :-|.