It just seems that 4 was scaled down for what seems to be relic raiding. If you look at 3.5 maps (continuity notwithstanding) the area between Avendell and LA is a considerable distance and seems to be expanded in 3.5 with that whole area that you can get the periannath gear and falme. Not to mention giant mountain and undermountain.
I know improvements are winding down with the coming of NS5 but I am sure NS4 will be played for some time after the debut of NS5. Don't get me wrong the planar areas are great, but some actually like running around killing and exploring without having to worry if your spot or listen isn't high enough to deal the latest batch of cheesy poofs.

There is I am sure many more maps waiting in the wings that have been made but not implemented.