We have cruised in a Cat 3 twice - both late August. I booked the Cat 3 well in advance both times - about 18 months in advance in order to get the best price and to get one of the 4 mid ship Cat 3.
Both times I checked the price of a Cat 4 at the same time and the Cat 3 was about 50% more.
Yes, you do get a much larger room with 2 bathrooms and several other added benefits but not everybody needs or wants the room and other benefits. Also, there are less expensive ways (2 adjoining cabins) to get the extra space and bathroom if money is the issue. If money is not really an issue, go for the Cat 3.