"20 category 4-5 per year since 1970? It is a big year if we have 20 per year of all categories, let alone 20 category 4-5."
I shouldn't have paraphrased...
"While studies have not found an overall increase in tropical storms worldwide, the number of storms reaching categories 4 and 5 grew from about 11 per year in the 1970s to 18 per year since 1990, according to a report in Friday's issue of the journal Science."
Is this the article you referenced?
http://www.sciencemag.org/cgi/content/full/309/5742/1844
Table 1 in this article says 20 cat 4-5 hurricanes in the North Atlantic in the period 1975-1989, and 25 cat 4-5 hurricanes in the period 1990-2004. That would seem to work out to be an avg of 1.33 per year during 1975-1989, and an avg of 1.67 per year during the period 1990-2004.