Just to reiterate what Brown said, media manufacturers play games with the numbers.
4700 MB does not equal 4.7 GB.
1024 bytes = 1 kilobyte
1024 kilobytes = 1 Megabyte
1024 Megabytes = 1 Gigabyte
What they're doing is basically listing the number of bytes contained in the DVD:
1024 B x 1024 K x 1024 M x 4.38 G = 4,702,989,189 bytes
But since your computer uses the correct increments for filesize, it'll tell you that the maximum amount of storable data is 4.38 gigabytes.
(That was probably overkill, but what the hell...)