A four inch washer to bolt a wood box to your floor board is overkill. Just get eight fender washers for whatever size bolt you're using and you'll be more than fine.

Why the hell do you need that?

Really, I doubt they make them. They would sell about 8.

Likely you'll need to have a machine shop make some.

BlackFriday

Just use drywall screws into the floorboard.

I am trying to figure out where I can buy a 4 inch outer diameter washer. I have been searching, and have had no luck finding any. I only need 8, because i am bolting my sub box to the floorboard in my car. Could someone please help?