Low Voltage Shunt Regulator
Datasheet

R1 is chosen to set Vref on the TL431 to its nominal value of 2.495V.  The output of the TL431 is dependent on the values of R1 and R2.

R4 is chosen to protect the Piezo from seeing more than 7V which is its limit.  To figure out the value of R4 you have to consider the voltage drop of Vds on the FET and the current the piezo draws.  From this calculate R4 to produce enough of a voltage drop to keep the voltage across the piezo to 7V or less.