You could have 4" hose and still have the same lift, what you need to know
is how long from placing the wand head on the carpet thus producing a seal it
takes to evacuate the recovery tank and hose to reach peak suction.
What everyone fails to realise is that if the wand is correctly sealed on the surface
of the carpet, it does not matter whether you have 2 x 3 stage in series or parallel,
or a truck mount or 25 x 6.6" configured however you like. Research has proved you
will always have a maximum of approximately 53 Cubic feet of air crossing that wand
carpet interface. The only way to raise that figure is to break the seal as in lift the wand
tilt it or fit a glide, at this point you are losing lift.