HD is a reference to the broadcast resolution. Anything higher than 720x576i resolution is considered HD.
MPEG-4 is a type of compression that is used to convert the uncompressed video which can be many hundreds of Mbps into something that can be transmitted through terrestrial transmitters or sat or cable delivery.
MPEG-4 has a number of different variants. Whilst the older Part 2 ISO standard was used in some storage formats (divx codec), the modern broadcast form of MPEG-4 is the Part 10 or AVC standard (also known as H.264) which will eventually replace the older common broadcast MPEG-2 codec due to its improved efficiency.
For now most MPEG-4 use in broadcast is generally limited to HD channels but eventually SD channels will also change over from MPEG-2 that is common these days.
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