What is the purpose of this bias resistor?
For Q1 to be off (where 'off' is defined as less than 250uA current) the gate-source voltage must be less than 1V (according to the datasheet value of Vgs(th). Since R5+R6 || R4 = 13.33K, and the gate sees 100/120 of the voltage, up to 90uA of leakage would be acceptable. That's a pretty conservative design considering the leakage is rated at Tj=125°C, unless you have some reason to expect very high operating temperatures (perhaps this is intended to operate in an under-the-hood environment). 

If R5+R6 were lowered to 11K/2K it would work similarly, but the zener would see a lot more current at high input voltages. With 60V in, R6 would be dissipating 1.6W vs. 0.24W in R4 plus 0.14W in R6. If you change the ratio of R5/R6 then it affects how much gate voltage is on Q1 with a low input voltage, and you'd have to analyze how that would affect the circuit (perhaps Q1 would burn up during cranking, for example).