4D is DG's name for their improved digital recording process which they began using about 1992 (it's standard redbook CD). I used to think their sound was generally pretty mediocre, but since about 1992 it's gotten a whole lot better. Just about any of the Boulez-conducted recordings, especially the ones with Chicago and Cleveland, are excellent musically and sonically. I especially like the Ravel piano concertos with Boulez and Zimerman--a great recording.
