That list is pretty terrible. 4 runners of the Boston marathon got brain cancer over ten years? Over 20,000 people finished the Boston marathon last year (http://www.bostonmarathon.org/2009/cf/public/statistics.htm). Even if we assume it's the same 20,000 people running every year for ten years (highly unlikely), an incidence of brain cancer of 4/20,000 is the same as the incidence in the general population 15-20 / 100,000. (http://www.oncologychannel.com/braincancer/index.shtml). So, 4 people is about what you would expect. Given that it's not the same people, it's probably a good deal lower than the rest of the population.

It's pretty difficult to take anything on that list seriously if they're going to blatantly misrepresent statistics like that.