Adjusting speed of a kitchen range hood motor
Simply put replace the 4" pipe with 6" pipe. 4" pipe is just too small for a range hood. This is going to be the only realistic way to make your range hood more efficient.

To put this into perspective....


The air movement volume is based upon cross sectional area.
The 4" pipe cross sectional area is 12.56 square inches.
The 6" pipe cross sectional area is 28.27 square inches.
The 6" pipe can thus handle more than twice as much air flow
Bends, corners and outside vent covers for 4" pipe are going to be much restrictive than corresponding 6" components. 


So do the job right rather than making a science project out of modifying your range hood motor speeds!!