> The question is what do you think if I just put a 4.7 zener in
> between the Vdd and Vss?

You'll need a series resistor too. It needs to be small enough to
make the zener break down (otherwise the excessive Vin may
get to the PIC and kill it), and large enough to limit current through
the zener when/if it does break down

Probably 5mA would be enough through the zener to keep it
regulating, but you'd have to check its datasheet, and also have a
think about how high the careless Vin might be to determine the
resistor and zener wattage rating

http://www.opamplabs.com/zenerreg.htm

For example, enter 10 (volts in), 4.7 (zener voltage), 10 (mA load)
and click Solve -> 265ohms 0.1W resistor, 0.1W zener

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