Thank you for your question on the model QT130E exhaust fan. This fan will handle a bathroom up to approximately 120 square feet with an eight foot ceiling, when installed with six inch rigid duct work. When you reduce the duct work to four inch, you can run no more than eight feet straight of rigid duct work without losing CFM. Any elbows in the duct work or the use of soft flexible duct work can cause more loss of CFM. Reinstalling with six inch rigid duct work should show you a big difference in performance.