Let's see if I can explain this properly.
Flipped-up my WH deck and measured from the underside of the
deck down to its lower edge. The deck is 4 3/4" deep and the cutting
edge of the blades is appx 4 5/8" from the underside of the deck.
So, I can't see the blades from the side of the deck as they
don't extend below the lower edge. It would be unsafe if the blades
extended below the lower edge.
Not sure what you mean by saying "the blades to be 4 inches
below the deck."
In any case, for a 4" cut the front casters have five 1/2"
spacers below the arm. For a 4" cut the rear roller pin position is
the lowest of six holes.
Jack