Well, the way you would do this is to get PR5+ sites to link to your PR4 sites.  This seems how you keep getting your PR to increase most readily.  Although, you could get a lot of PR4 sites to link to you and if enough of them do, it should likewise push up your ranking.  I really don't think a site's rank makes a bit of difference.  It is more important that you are appearing in the top 10 for your keywords since this is what people search by (not page rank)...

They sometimes do but not always. If you have good keywords you will often find yourself ahead of the higher competition. High PR get's you crawled more by Google though, and it's easier to get links from other sites when you have a high PR. People always want to exchange links and stuff when they see that you have a 5+. Also, people that want to spend money on your site to advertise, will look for a high page in order to boost theirs. That's my case right now. I am looking for websites to advertise on and I am willing to spend some money, but I won't spend it on some site with a 2 PR.  I want it to be high so that it can boost my PR.

Hehe, yeah Google's pages are often ranked up there, but so is Yahoo most of the time. I read somewhere about some others but can't think of them right now. Wouldn't it be sweet to see LP with a page rank of 10?!

I am not sure as to how much to spend on a 5 PR site, because it depends on the traffic the site is getting. It is possible to have a PR 5+ site and only get a hundred or so hits, but then you might find some page with a 2 PR and get way more hits. What you need to do is make a template and email quite a few sites and ask them for prices or a link exchange. Modify the template just a bit and send it out to as many sites as you can find.

To find topic relative sites I would go to Google and type in phone "advertise" or something like that. It should find you enough information. DMOZ is also helpful, because you can go to a relative category and click on that green dot on the bottom right of the results. Once you click it, you will be able to see all the sites with the PR right next to them. It shows the top ranked sites first, and should make your process a lot easier.