...two posts by ciml, with my comment in brackets:
To get PR4 [from PR3] you need about "n" links from PR4 pages, or about "n x n" links from PR3 pages, or about 1 link from a PR5 page. I say about, because the number of links on the linking page make a difference, as does whether the linking page is a at the top or bottom of it's position in the Toolbar.
People disagree over whether "n" is about 6 or 10.
And, in answer to a question abt 3 PR6 links...
I'd expect roughly PR5 from three links from PR6 pages.
If the same rules apply going from PR5 to PR6, this suggests "n x n" PR5 links, or somewhere between 36 and 100... consistent with what Pegasus says.