this is one of the greatest misunderstandings in storage media/devices:
In DVD media and in harddisks 1GB=1000MB=1000*1000KB=1000*1000*1000bytes
In memory dimms and in cdr disks 1GB=1024MB=1024*1024KB=1024*1024*1024bytes. This is also the "true" definition of GB because it is a power of 2.
So the GB for dimms is slightly bigger than the GB for dvd media and hard disks. Same holds between the two definitions for "MB" and between the two definitions for "KB" .
Nero does the calculations as in memory dimms so the 4.7GB=4700000000B as it writes in the label of DVD-R discs are in fact only 4700000000/(1024*1024)=4482 MB or about so.
The slight error in calculation is because dvd-r hold somewhere between 4.7GB and 4.71GB (i use the first definition for GB here that is 1GB=1billion bytes) .