Using Supercapacitors with Motors - DC/DC converter?
You need to get some spreadsheets going to understand your system.

Your chosen motors may well be capable of drawing 400A at low speeds. However, that doesn't mean they are drawing more power than when used at high speed, as the voltage will be lower. It also certainly doesn't mean you should be supplying 400A.

Your should look again at DC/DC converters. You need to limit, to control, the current into the motor at low speeds, in order to control your acceleration. A DC/DC converter will take your high battery voltage at low current and convert it to a low voltage at a higher current when the motors are at low speed. 

If you simply slam in 400A per motor, I would expect the high torque to be able to spin wheels, which would waste energy and lose you control of the vehicle.

You do not need to connect all the motors in parallel, you could connect them in pairs and use a 1000A converter for each pair. 

Your supercap 'solution' increases the weight and cost of the vehicle (probably bad) and doesn't solve the real problem, which is obtaining controllable acceleration from standstill.