sometimes its in millions so it all depend on what is the quality of your page n how unique and qualitative is your content on the page
I'd say 70-80 atleast (Unless there are some PR8-9 sites giving you back links), and your website being opened for a couple of months of course.
It actually has no particular measures. Sometimes few backlinks from PR 5 and 6 will help your site have a PR 4 and sometimes keywords do the magic.
my other site 170-link PR-2 1 year old (classifieds ads), 110-link PR-1 10months old (classified ads), 30-link PR-3 6months old (directory). Based on these alone its not based on the link. maybe the "wiki" domains means something from google. Among the 3 this actually has the less visitors funny right.
it also depends on the value of websites from which you got the backlinks to your site. Those websites should be high valued.
I think you need high quality back links of good page rank sites for getting page rank improvement and also do the work of web directory. it will also increase your page rank.
engkusplus has posted a nice table there. I don't see in PR as much importance as I used too in the past though.
Not all the backlings are of same value. PR4 depends on the value of your backlinks. If you can provide quality backlinks then it will take less time to get PR4.
Depend on so many factors, like dofollow links from high PR site, good content, less error pages etc.