It is difficult to specify how many back links, but getting quality back links from high page rank site having different ip address and do follow attributes will help a site to get high page rank. Google considers not only backlinks in determining the page rank, it also considers some other factors like content, traffic, internal link structure and so on. I Thinks so
I have seen many websites which have zero backlinks but good page rank..If website have original content it must help to gain pagerank..
relevant backlinks supports your niche,.. actually i'd prefer seeing that the contents[poss,categories,etc] relevancy to title and domain name is prior consideration for high pagerank... then we build backlinks to the contents/categories/subpages, not only the main url.. let the search engine define your url naturally by metatags.. it is the best n safest way in building backlinks if you want long-lasting PR.. yes, we can't do this instantly by ourselves, unless you have tactical team to do the details..
Getting PR4 in Google page ranks its not easy. Yeah it depends on various Scenario. I think you need 100+ PR5 or above back links.
First google change its method and you do not know how many baclinks your site required for getting pr 4 according to my knowledge one site have 300 quality backlinks and minimum 10000 traffic daily
Quality of backlinks is so important... If there is no one click on your back-link or your back-links is not meaningful, it's not good.
With so many strategies that explained here.........it also depend on the existing PR i.e gradually increase .... from PR3 to PR4 or from PR2 to PR4, depend on the quality and relevancy of the backlinks.
Shoot for quality backlinks via article directories and web 2.0 properties like Squidoo and Hub pages. Every backlink counts. Take your hyperlink and put it where people will see it.
Here you can see how many backlinks do you need for the pagerank you want - Google is according pageranks with this formula: linksmanagement.com/view/google_pr_formula