So theoretically each rank in PR is a factor 10 higher to get. So lets assume you need 1 of the exact same strength of link to get PR1, then you need 10 of theese to get PR2, 100 of theese to get PR3, 1000 of theese to get PR4, 10000 of theese to get PR5 etc. Though factors like relevance of links and content, age of domain, consistent posting also comes in play. So all in all just build consistent relevant links to a website with always new content, and you should get PR4 relatively quick.