Whats the cheapest but strongest 4x8 I can use while only supporting the ends lengthwise of the board?
There is no magic flat material that will have that kind of strength, short of some sort of honeycombed steel or carbon fiber, and I'm sure you don't have the budget for that.
Maybe you could use scaffolding planks, but again, budget.
You'll need to frame that out with joists
And probably the cheapest will be conventional wood framing.
Since the critical dimension will be across the long length, you frame it out with 8 foot long &quot;2x6&quot; dimensional lumber oriented vertically for span strength.  I would follow the 16&quot; spacing rule (though it will really be 15.5&quot; center-center spacing), so that will be four 2x6 joists.   Two hug the edges of the plywood and two are spaced evenly between.
Now, we need something to prevent the joists from leaning sideways.  So some more 2x6 spacers go between the joists, wherever convenient.
Now, myself, I would screw the plywood to the joists with deck screws.  About every foot should suffice. You can nail it if you really want to, but you'll have an easier time disassembling the wood for reuse if you use screws.
The whole package will be 5.25&quot; + the thickness of the plywood. So 3/4&quot; plywood = 6&quot; total height.