Automotive Load Dump Protection Circuit
Yes, (their) R5 &amp; R6 can perform that function, but not as well, depending on your constraints.
To ensure the 2nd FET turns on well at low VBATT, you need to ensure that your R1 &gt;&gt; your R13; this ensures that at low VBATT, most of the supply voltage (VBATT*R1/(R1+R13)) appears across the VGS, turning the device on. However you can't make R13 too low, or at higher battery voltages, you will have too much current in the zener (Zener current = (VBATT-6.8)/R13) which could overheat it. You can't make R1 too high, or the turn off time will become too large.
Adding their R4 provides good turn-off time (approx. proportional to 1/(R4+R6)), but doesn't overpower the zener.
In fact, you could use R4 and eliminate R5 with no noticeable effect.
Be careful because not only are load dumps possible (and are relatively slow -- &lt; 1 ms rise time), but other sharp high voltage transients can occur (e.g. double battery jump start @ 26.5 V, impulse noise). C1 also slows down the turn-off of the circuit at high input voltages (approx t= C.V/i = 100n*6.8/15mA = 40 us); 10 nF would be safer.