An interesting sets of facts about Google PageRank, originally posted at netconcepts:
“Each web page within a website has its own PageRank score. PageRank scores run from 0 to 10 on a logarithmic scale, meaning that the gaps between the integers increase logarithmically the closer you get to 10. So, for example, the gap between the 3 and a 4 is quite small, whereas the gap between 7 and 8 is huge in comparison. As such, boosting your PageRank from a 3 to 4 would be quite easy, and going from a 7 to 8 would be quite hard. Another logarithmic scale you might be familiar with is the Richter scale. As you probably know, a 5.5 on the Richter scale isn’t such a huge deal, whereas a 7.0 is a very big deal indeed.”
I think going from a 4 or 5 PR is probably very similar to a 7 on the Richter scale – better have your server ready for the jolt!