That's right. You can take the same amount of material per revolution
either way. But at 4x the speed (given the same pulley setup) you can
remove 4x the wood per time period. Of course, there are certain
non-linearities in the way controllers work, but that is the basic
proportion. For an understanding of the practicality of how horsepower
works, watch a Mike Mahoney video and see how fast he roughs a bowl with his
3 hp Vicmarc lathes. Yes, horsepower really does cut the wood, as does
torque!

As you turn down the speed, the horsepower decreases linearly from
your
example, and thus the torque is constant from the above equation. If I
understand all this right, I can take the same heavy cut (amount of material
removed per revolution) at full speed or 1/4 speed, I just have to adjust
how fast I move the tool along the tool rest. Just to be clear, I'm still
talking about electronic speed control here.