http://club.cdfreaks.com/f33/actual-size-dvd-r-4-7gb-139256/
The difference is that there are different definitions on megabyte and gigabyte. When the program shows how many megabyte the disc can hold it uses the same definition of megabyte as Windows does, 1 megabyte = 1024 X 1024.
Disc manufactures uses the same definition as producers of hard disks does, 1 megabyte = 1000 X 1000. But in reality it's the same amount of data that they are talking about. 4483 megabyte = 4483 X 1024 X 1024 = 4700766208 bytes = 4,7 GB.