Question: how loud is 2 sones?
Answer: Sones are not decibels or volume, but rather how sound is sensed. Sones are a linear measurement, like inches. Doubling the sone value is equivalent to doubling the loudness (i.e. one to two, two to four is doubling the sound level twice). Your 4-sone fan will be more than four times a noisy as a 0.9 sone fan.