It depends on what diameter of which part of the unit you are measuring.
The inside diameter of the cylindrical chambers where the fan blades rotate are 3 inches and 4 inches. Since there are so many variables in the measurements that are so close in size, it is generalized to the two categories for ease of reference.
In other words, anything from 3” to 3 15/16” is categorized as a 3 inch product. Anything from 4” to 4 15/16” is categorized as a 4 inch product.