How do i count a fluctuating 22.1v (11.5v) pulse from reed switch with Arduino?
I'll need a little more information before we know what to do. What do you plan to do with the 4.7V? Is it to be a power supply for some logic parts? i.e. does it want to be a DC signal? if yes, then what is the load on that power rail? If you are powering some simple logic parts, then its probably pretty small, maybe a few milli-amps. 

You need to know the load current so you can pick the resistor value to put before the zener diode as the resistor will have both the load current and the diode current flowing through it.

As for the capacitor, it depends on the AC response of the zener diode. The capacitor is a low pass filter so it will smooth out the voltage changes that the diode will see. the 3db frequency for the filter is 1/(2*piRC) and you typically would choose a frequency that is 1/10 of the frequency you are trying to filter out. so this depends on how often the signal on the 24V line is changing.