Is the upgrade to a Cat V or a Cat T and from what stateroom level currently?
We did the Dream 4 night in a Cat T connecting Cat V. If the upgrade is to a Cat V, I would probably not do the upgrade for a 4 night cruise unless you really will use the concierge facilities. It the upgrade is to a Cat T, I might consider it, but only if I knew we would use the stateroom on that cruise.
When I booked my Cat T/Cat V combo, the entire upgrade was only about $800 more than the 2 Cat 4Bs I had booked so it made a lot of sense to book. I was traveling with my parents and sister and knew we would use the extra space a Cat T gave. We basically used the main space of the Cat T as the living room and the Cat V as a second bedroom. My parents spent a lot of time in the room and on the balcony to make it worth our money. My dad and I also visited the lounge frequently during the cruise.
Although I enjoyed concierge, I would have to consider whether or not to book concierge again based on the people in my traveling party for a 3 or 4 night. There are some that the concierge space would be wasted on unless the price was really good, but others I might be willing to pay more since they would spend more time in the stateroom and use the concierge services.