How to hit PR 4

There are many ways to get a PR 4 to your website
1. Given the assumed dampening factor of 0.85 and the base-6 system for each PR level a single PR4 and a single PR3 would do the job, as long as the link to your site was the only followed link on each of those pages including internal links

Other ways to get there:

Get 7 Links from PR3s
Get 42 Links from PR2s
Get 252 Links from PR1s

Really that's all useless though, because it's highly unlikely you'd be the only link on a given page. If there are other links on the page, you'll need to take the above numbers and multiply them by (links + 1) to see how many you'd need. For example, if those pages all had 10 links on each of them (plus yours), you'd need to multiply by 11 to see how many you'd need. Using those numbers, you'd need:

You need 77 PR3s
You need 462 PR2s
You need 2772 PR1s

Those are probably more reasonable numbers, since most pages have plenty of internal links to compete with your link for the juice.