Short answer: MPEG2 was developed in the early 90's and evolved from the earlier MPEG1 standard. MPEG4 became a standard in 1999. Satellite bandwidth is limited; moving to MPEG4 allows broadcasters to use their bandwidth efficiently, while improving quality. Unfortunately, due to the high penetration of MPEG2 hardware in places like the USA, it has made it hard for broadcasters there to switch to MPEG4. The impact has been that they have to apply high compression to their MPEG2 broadcasts to maximise bandwidth, affecting the quality of the broadcast.
A, perhaps over-simplistic, example - take a movie, compress and encode it in MPEG2, and it will take up a 4Gig DVD. Take the same movie, compress and encode it in MPEG4, and it could fit to a 700 meg CD, with quality equal to the DVD. DivX is probably the most popular example of an MPEG4 codec.
For the long answer, see
Tom's hardware .
Disclaimer for the purists: over-simplifying to make the answer short and understandable.