Don't play naval architect. 4mm okuome is not only thinner it is only 3 plys. 6mm okuome is 5 plys. That means it's much stiffer.
You can protect and strengthen the bottom without adding too much weight by laying on a layer of 6 oz glass.
Even if you can bench press 500 lb. you will not be able to drive a hull with less than a 12 foot waterline to anything but a pedestrian pace.

: I'm about to order wood for my first kayak (Wood Duck 12) and due to the many
: underwater logs and stumps in my area, I was thinking about using thicker
: plywood. The plans call 4mm ply, but I am thinking about using 6mm. Is
: this a good idea and will 6mm ply bend as much as 4mm?
: thanks