The problem is that more than 80% of U2 fans are not at home at 10 pm on a Friday night. Most are 25-40 and are usually out at that point. I'd like to know the time and day those other music specials were on because that has a much bigger influence on the number of people that will be watching than the actual popularity of the artist.
In addition, only 2.4 million watched the POP music special back in 1997, so if the figure is 4 million this time around, thats a 70% increase in the number of people watching. Also realize that the latest studio albums by the other artist have not even come close to selling 11 million copies worldwide like U2s ATYCLB has.