Uhh, even EchoStar III could broadcast MPEG4, so it's not the purchase of Rainbow-1 that allows MPEG4 content.
Think of the Sat's as digital pipes with error correction added to the signal, it can be sent as WMV9, MPEG1.5 (D* was this for a time), MPEG2 or MPEG4, to the Sat it don't matter...