I'm not sure I completely understand what your trying to say but here goes.
1. It all depends really. Depending on resolution and quality of the video and so forth. 4.7GB is alot though.
2. Ah you see... only DVD-RWs can be recorded on more than once. Although there is one exception. If you haven't finalized the DVD then you can record stuff to it again just from windows explorer (if your using windows XP that is, I wouldn't be best to tell you on other OS's because I use windows most of the time, even though I prefer macs). But once it's finalized then no more can be put on it. But some DVD players and such only except finalized DVDs. Only a DVD-RW can be written over multiple times.
I hope I was answering the right questions. Hope it helps.