You likely found this page by searching for From PR3 to PR4 in Google, and that is a pretty good indication, that you are searching for, and reading the wrong SEO articles. There quite simply isn't a simple answer to how long it will take, or how to get there. Given enough time and effort however, well who knows?
Getting there slowly
For some it may take months, for others years. So its hard to tell how long it will take. Try not to obsess so much about PageRank, and look on the rankings of your pages instead. This can be done in Google webmaster tools.
However if you still want to learn more about the things which may effect your pagerank, then you should really check out Brugbart's SEO Tutorial.