lol It may be old, but I feel the need to do this
The DVD actually does hold 4.7 GB. And can only be overwritten over to a certain extent. The DVD process adds files due to the writing process, so depending on how the DVD is burned and the media on it, will determine how much space is used
Burning files as 'data' discs, allows for the full amount of space, because it will only be used to read the files themselves, and not in any special sort of way
Media discs to be used for play in DVD players and the like, have to be burned in certain ways with special files. Which is why those are calculated by the amount of time the movie contains, rather than the space. It requires a certain configuration file, to allow it too be read in the DVD player
Also, here's a need little table I did just for you guys xD Though the same thing is probably everywhere else
1 Bit: Nothing, base for storage
1 Byte: 8 Bits
1 Kilobyte: 1,024 Bytes/8,192 Bits
1 Megabyte: 1,024 Kilobytes/1,048,576 Bytes/8,388,608 Bits
1 Gigabyte: 1,024 Megabytes/1,073,741,824 Bytes/8,589,934,592 Bits
1 Terabyte: 1,024 Gigabytes/1,099,511,627,776 Bytes/8,796,093,022,208 Bits
1 Petabyte: 1,024 Terabytes/1,125,899,906,842,624 Bytes/9,007,199,254,740,992 Bits