Every time I see someone selling or trading a link bragging about it coming from a PR4 page, I'm not sure whether to chuckle or boggle. "PR4?" I think. "That's supposed to be impressive?" I guess it impresses some people, even if it doesn't impress us all. Which raises the question, what is "good" PageRank? I can imagine a few possible answers:

1) Good PageRank is anything higher than what you used to have. So yeah, PR4 really is good if you used to have PR3.

2) Good PageRank is however much it takes to move to the top of the search results. Keep in mind that higher relevance can overcome lower PageRank. A PR4 site that Google knows is highly relevant to particular search terms will come up higher than a PR7 site that's about something else but happens to mention those terms.

3) Good PageRank is however much it takes to get people to trade links with you. Of course, the higher your PR, the more likely you are to get lots of trades and trades from high PR sites.

4) Good PageRank is however much it takes to get people to buy links from you. But of course, higher PR will lead to a higher link price. (Just be sure not to claim to be selling PageRank, nor to tout your PageRank alone as the reason to buy the link--Google doesn't look kindly on people trying to game the system by selling PageRank).

So sure, be happy about your PR4. I'll be happy about my PR7.