What is purpose of components in this circuit
D2 keeps the MOSFET gate voltage from getting too high. R6 drops the voltage that doesn't appear across D2 and Q2. The MOSFET gate is rated for 20V so 6.8V is very conservative for the zener voltage. 

R4 pulls up the drain of Q2 to close to the input voltage even if there is a bit of leakage in Q2. Maximum leakage allowable is about 1.2V/13.3K = 90uA, which again is conservative (50uA maximum at 125C for the ZVN4106, but at 48V rather than the 60V it could see).

A similar thing could be accomplished by reducing R5 and R6 to 11K and 2.2K respectively, but it would waste as much as another 3-4mA under normal operating conditions.