I'm not sure I understand what you are asking.
If your original DVD is 4GB, and you are putting it on a single layer recordable DVD (which holds 4.35GB), why would you have to process the video at all? This should be burned on the disc with no loss of quality.
The usual meaning of transcoding, when talking about compressing DVD's, means that you are using the residual data in the form of DCT coefficients and requantizing them. Some of the original data is discarded in this compression process. Motion vectors are not used by a transcoder, and so a great deal of information available in the video is not utilized properly.
If you use an mpeg2 encoder, like CCE, or tmpgenc or HCenc, all of the information is recalculated in order to make a smaller, compressed copy and so the encoded copy will tend to be much more accurate than something produced by a transcoder unless there is only a trivial amount of compression necessary.
I don't know if this answers your question or not. Perhaps you mean something else when using the term "transcoding".