Just like trucks, some of which are almost exactly 4' between the wheel humps. The Toyota Previa (late-80s to mid-90s) and early Dodge Caravan vans can easily carry full sheets of plywood.

That it is possible to put a full sheet of plywood in a minivan has nothing to do with the design criteria for the first minivans. The design criteria for Dodge could have been fit a family of six and have space for luggage and a pet carrier. It is surprising that fitting 4×8 sheets were an explicit design criteria.

They fit exactly laying flat, just like some pickup trucks. Not an inch to spare. That's not a coincidence.