Simply they both are the compression technology used by dth operators.
Mpeg 4 is advanced form of mpeg 2. For the same quality of video mpeg 4 requires less space than mpeg 2 , because of this dth operators can provide more channels if they use mpeg 4 instead of mpeg 2.
For more detail you can search in the forum or simply google it.
To get these mpeg4 channels you need a set top box that supports mpeg4 format.If you are using mpeg 2 stb you can not get these channels.Currently most of the users are using mpeg2 stb.
I hope this helps..