MPEG III was going to be a higher quality encoding for HDTV. It transpired after some studies that MPEG II at higher rates is pretty good, and so MPEG III has been dropped.
MPEG IV is aimed at the opposite extreme - that of low bandwidth or low storage capacity environments (e.g. PDAs). It is based around model-based image coding schemes (i.e. knowing what is in the picture!). It is aimed at UP TO 64Kbps.