The report considers patents held by the big four - saying that ATI has 569 US patents, compared to AMD's 7,574. But even though the ATI number is small, the patents underline a number of technologies that depend on them. Some are "highly influential".
Nvidia only has 295 but they're pretty important too in terms of interdependencies.
It considers the effect of other possible takeovers, too. There's a link to the PDF from this page. µ