I have a circuit that's supposed to cut off at <4V. It instead caps the voltage at 3.3-ish and reduces the voltage linearly below that. Zener - 3.3v Source - 5V +-5V
A zener diode is not a perfect stop for current, there is still some leakage. Further because you are using a darlington pair type setup there is a very high beta allowing just even a little bit of current is still enough to tun them on.
Also this is just my best guess, your question is not very clear.