Electrical – Calculating parasitic capacitance of serial zener diodes

Please define "optimum release time". Does that mean that you want the fastest possible release time?

If so, simply allow the flyback voltage to go as high as possible. In other words, pick a zener value that is somewhat less than the maximum voltage that your transistor can handle.

And remove catch diode D2 - you don't want it if you need fast relay release.

You need a resistor between the switch and zener, and a resistor between the Vout point and ground, like so:

^{simulate this circuit – Schematic created using CircuitLab}

With the switch up, the Zener will limit Vout to 4 volts.

With the switch down, the Zener will not conduct, and Vout will be 3 volts (less a bit, due to the current drawn by R2)

Without R1, when the switch is up, infinite current would flow through the Zener diode as V1 tries to deliver 5 volts, and D1 tries to hold the voltage to 4 volts.