[Dale(294engr]]

Assuming you've eliminated mechanical drag: bearings/bushings/chain drive

what is Full Speed current draw of ea motor isolated alone, not driving gears?
is it the same at FS reverse?

Is 4A higher with all wheels off floor and full throttle?

If not repeat test off floor.

start removing things until problem goes away. (4A decreases to equal OK side) remove chains one at time test,...

electrically disconnect ea motor, test, (swap, test)
physically remove ea motor (leaving one of two or three)

make sure 2 motor H-bridge drivers driving same gearbox are balanced. (sharing load -- not contributing to it)

photo from bottom may be instructive.. Good Luck..

Safety issue.. your battery is vulnerable to being pierced by robot collision!!
consider welding/mounting a thick aluminium barrier protection!!