Since Pagerank is an expensive computation, one effective optimization/approximation is to cluster pages into groups and then compute the Pagerank per group. The group-scores are then re-distributed.
I am not sure whether this is what is going on, but it is a possibility.
Edit: just found one that is a 3, but it has a trailing slash while the others don't (so it might be considered a directory and computed seperately.)
It also just occurred to me that my xml sitemap assigns a priority to each page, and the ones with a PR of 4 are the ones with a 0.8 priority or higher. There's another top-level page with a 0.5, but it has a trailing slash, and it's a wordpress blog so it has it's own sitemap. So I think that 4 might be independently calculated.
Not trying to toot my own horn, but you might find the bookmarklett on this page useful, it certainly made things much easier for me: http://nfriedly.com/pagerank