4inch fans are normally very quiet 30+ db range
most fans do not work well with standard cheap controllers they hum
these things work just like a light dimmer switch
so some folk use variable transformers (variacs) instead to slow the fans down without the hum/buzzing noise but variacs do not tend to offer as much control over the speed, also running the fan at a lower speed than it was intended can in some cases shorten the fans life
if your issue is with noise, best thing to do is put your fan in an insulated box, rockwool inside the box works well to trap the sound
also sound insulated ducting will also reduce the noise , much of the sound, is the rushing sound of the air through the ducting
any fan over 40 db is pretty loud, fans rated at 30db can still move lots of air, and are no louder in a room than a small house fan heater
peace