Actually its not depends on Backlinks.there are lots of sites in internet having millions of backlinks bu they dont have even PR1
The best backlink checker is Alexa. Make an account, submit your site, claim it - that's all. You will see all the backlinks you have - and it is really the best and the fastest backlink checker; all your backlinks are appearing in a few days, if they exists!
So far my knowledge is concerned, there is no specific count to achieve PR4. There is more complex algorithm involves calculating the PR of a site. The all you can do is keep posting quality contents, try to get link back from higher authority sites and follow White Hat SEO techniques.
Hi, It depends on the quality back links that too not less than 500 to 600 backlinks from quality sites.