Well the best way I can put it is that when a disc says it has a 4.7GB capacity it's kinda telling a half truth. What it means is that it has 4.7x10^9 (ten to the power of 9) bytes of capacity. So if you want the capacity in the GB that we know and use, you have to do 4.7x10^9 divided by 1024 (the number of bytes in a KB, the number of KB in a MB and the number of MB in a GB) three times and you end up with a capacity of about 4.38GB.

So in reality, you can only fit in about 4.38GB worth of data on to the disc, or about 4485MB... though my discs say 4489MB (an extra 4MB ).