c/p from another site...

MPEG4 and 8PSK have nothing to do with each other. MPEG4 is a compression/encoding method and 8PSK is a modulation method. To give you an idea, in the analog world, AM and FM are two different modulation methods. In the digital world, there are many modulation methods and they have different applications. For instance 8PSK and QPSK are used for DVB/DSS satellite communications, QAM, and PSK are used to deliver digital cable and internet over cable, VSB for digital TV over the air (VHF/UHF), CDMA, WCDMA and TDMA are for cellular communications, and OFDM commonly used for WiFi. I believe WiMax will use OFDM as well. Modulation methods are carried at the RF layer while the compression/encoding methods are carried at the Presentation layer so they are independent from each other.
MPEG2 and MPEG4 are two of many compression/encoding formats used to deliver digital media. You can carry MPEG4 or MPEG2 compression formats over ANY of the modulation methods as they are independent from each other. So for instance, you can carry MPEG2 or MPEG4 over 8PSK or QPSK, whichever you want. You can even do MPEG4 over QAM if you want, which is
probably what cable companies are working on doing in the near future. By the way, digital off the air TV uses 8VSB with MPEG2 compression. MPEG2 and MPEG4 can also be delivered via physical media, such as DVD, HD-DVD or Blue Ray discs. According to Lyngsat, all of the other HD channels are still in MPEG2 over standard DVB QPSK modulation, while some of Dish HD channels are in still in MPEG2 over DVB QPSK and others in MPEG4 using DVB 8PSK
modulation (former Voom channels). So essentially, what Dish is doing is borrowing 8PSK from an old company they bought a few years ago (Voom) and converting the rest of their existing network to this new proprietary modulation method.
By 2009 probably all transponders at both Dish and others will be carrying
MPEG4 rather than MPEG2. Even DTV is in the process of converting everything from MPEG2 to MPEG4, but instead of DVB, they use DSS which is DTV's proprietary protocol. That's why nobody can test them now. However, I've read that DTV is slowly moving to the DVB-S2 in some birds, still using QPSK modulation. Dish will continue to use QPSK until they convert everything to 8PSK and others will continue using QPSK for modulation. But who knows? Maybe others are in the process of licensing 8PSK to convert to it in the future. You never know. However, they're all moving to the more efficient MPEG4/AVC compression codec. This way they will fit more channels on the same transponder.
Another thing, let's not confuse encoding methods with encryption methods. Nagravision and Digicipher are encryption methods. MPEG4/AVC and MPEG2 are encoding methods. QPSK, 8PSK, etc. are modulation methods.

Modulation methods even have some subcategories, like in QAM you'll find 16QAM, 64QAM or even 256QAM. The higher the better. In the PSK method, you'll find QPSK, 8PSK, and even 16PSK. The same goes for VSB, which the most commonly used is 8VSB in digital TV today in the US.