Keith, I'm afraid you are giving out some very bad advice, in my opinion.
In particular this statement is absolutely wrong, and I hope that nobody will put any stock in it.
Do not link (or for that matter) allow links to sites with less than a PR4.
Today's PR3 site is tomorrow's PR8. Don't look at toolbar PR as a way to determine who to link to. Never, ever.
Everybody has a brain, and most people have at least a little common sense.
If a site is useful to your visitors, feel free to link to it regardless of its pagerank. To do anything less would be silly, imo. You'd be doing your visitors a disservice. You won't ever be penalized for linking to great sites. It won't happen, this you can count on.
And Keith, you are also wrong about affiliate links. Most of them do not count towards your link pop or PR at all. They are generally not direct links in order to be tracked by the affiliate program. Therefore they aren't useful for that purpose. Using affiliate links are great, just don't do it for PageRank or link pop purposes.
I don't know where you're getting your info from Keith, but imo, it's very outdated, and simply incorrect.