Hmmm. Interesting results.
Many of us have known for a while that PageRank is nowhere near as relevant as it was in the past.
It does sound a bit like they were comparing apples and oranges. On one side the PR4 paid links sound like they could have been coming from directories or possibly footer links, which have been discounted for some time. On the other side they were using links on PR0 pages and varying the hyperlink text. That means that they were not comparable tests.
I do not doubt their results, but their test may not have been valid. They should have varied the text on both PR links to see if the PR4 links then outweighed the PR0.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin