Automatic series/parallel configuration
The thought process of design should be to define all the input and output requirements with all the ambient stress environmental specs and assumptions needed and make a list of these specs.  It should not limit the specs to any parts or configuration or imagined implementation.  Think about what is really important.
I am guessing you need to heat 4 locations evenly with 5W like a coffee cup heater and it must work off a car or truck so what counts is the temperature and not just the voltage.
Now if you define the output in terms of power instead of voltage and thermal resistance with temp rise, you can imagine PWM with a regulated thermal sensed solution.
Design Problems:

FETS make lousy comparators due to wide variation of Vt=Vgs(th)
Old power FETS were typically Vt= 2~4V @250uA i.e. 3V +/-33%
They do make Vt=8.5V but never 20V . The trend now is sub-threshold FETs Vt&lt;~1V

BTW if I guessed what you were thinking. look at Mercedes heater-cooler dual cup solution using thermal heatpump chip to make cold one side hot the other