The price of 4 ga welding wire is not that much more than the 6 ga you should have to handle the current of the charger. 4 ga will have less voltage drop than 6 ga. You will have the advantage of the larger cable for the life of the converter. The last time I bought 4 ga it was about $1/ft.
Nobody on this Forum, including the OP will know the draw of the batteries and the possible current from the converter to the batteries. It is a dynamic number and could approach 90 amps. That is why the proper sized wire is stipulated to handle the power.
