Hah! Me neither. That's why I had to ask.Can you post a link to the case? I've never heard of a case stating fan size in inches. Anyways, 4 inches is closer to 100 mm.
doesn't seem to be 120mm...
seems probable that it's a 92mm fan. As ~102mm is closer to 92mm than it is 120mm
I used Paint to measure the rough diameter of the fan and the width of the case in pixels based on the stock image of the back.
assuming the case is 173mm wide (6.8") while the fan is ~144 px in diameter and case is ~265 px in width, the fan is ~94mm, which makes 92mm the likely diameter for the fan.