Ok, it looks just like waveform Y in figure 2, here:
http://www.tomco-inc.com/Tech_Tips/ttt38.pdf
that lists it as 14 and 11 volts, not 5. Hmm.
If it were 14 volts then a 20k resistor in parallel with the zener should just about do it.
If it's really 4 volts and 5 volts, then maybe a 120k resistor in parallel with the zener would work.
(assuming standard zener and 100k resistor on injector lead)