A 4gb DVD is what you call a DVD5 which holds around 4 700 000 000 bytes and that is 4.37 computer GB. This is either re-encoded or barebones.

A 7gb DVD is what you call a DVD9 (basically what you purchase in the store) and holds around 8 540 000 000 bytes and that is 7.95 computer GB.

I used 7gb to make things easier. Today most studios try and encode anywhere from 6gb to 13gb for single layer DVDs. I don't know about pornographers though, but I hope that helps.
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I hit her with the hammer on top of the head. She made a lot of noise and kept on making noise, so I hit her again.