This is not a good burn to judge, you should have burned a whole dvd. It is not possible to draw conclusions from a 670mb burned disk.
Speed at 4.55 is the reading speed, and the reading speed goes up from 3.75 or 4 to 8x speed. This has nothing to do with the burned speed of your fresh burned dvd.
To make your pc configuration optimal when burning a dvd you can do some things to improve writings quality.
- defragmentade your harddrive before burning a dvd
- stop virusscanner
- no internet activity
- as less programms in the background as possible (no spyware, etc)
All those things can help to make the burningsprocess a bit smoother.
Well burn a full disk and come back with a scan. Using the search about qualityscan / cdspeed tool won't hurt you. Those things have been asked and explained many times.