Let's say I have a page that has a PR of 4, and I'm trying to raise it. Would the only way to raise it be to get a backlink from a site with a PR of 5? Could I get 100 pages with PR of 3 and I'd still stay at four? If you get a backlink with a pagerank equivalent to yours there is no change?
I remember seeing a chart somewhere, it was like 1000 PR1 backlinks to get a PR4, but only 1 PR5. So, of course, the higher the PR the better. Do remember though that if theirs more than 1 outgoing link on that PR5 page the PR will be split up amongst the links, not sure by how much. Any yes, the more the merrier, preferrably with a good keyword anchor text to help you in the serps too.
It's some kind of logarithmic scale where it gets more difficult as you move up. I remember about a year and a half ago people were saying its scale was about 6, but who knows..