I have the 4 inside Boxes running for a very small percentage in the Quaddies on Sandown.

Lomaca.
Thats why I do Markets.
Supposing you narrow the Field down , like you say in Race 8 , to 3 runners.
If you think the top selection is only slightly better chance of winning than the next 2 , and the Market available shows your top selection quite short , and your other 2 at good Odds, it would be wasteful to back Only the top selection.
But you not a fool and you knew that.
Every individual Race has its own peculiarities and they must be accounted for.