I can definitely tell the difference between the two rooms. It is pretty much only the width of the twin murphy bed but it includes a lot more storage space since the cabinet extends that extra difference as well (or on the classic ships, there was an additional storage cabinet/chest). We have always done Cat 4s when it was 2A, 2C just for the extra room.
That being said, for our Alaska cruise next year I was in a Cat 5 sized room since just me and the kids were going, but when my DH decided to join us, rather than move to a Cat 4 for the extra space (we are now 3A, 1C), we opted to book a cat 11 across the hall instead and put my DS and DH in that room. That gives us extra room, two bathrooms, etc and actually came out cheaper than all of us in 1 Cat 4.