It's very difficult to give answer on this question. It's follow many criteria and no body knows it's just Google crawler knows. We just can assume that quality back links and quality work will improve SERP and PR, but we can,t predict quantity.
If you build 5 to 10 pr 5 backlinks then you will reach a pr 4 and remember the backlinks should be do-follow otherwise you won't get the link juice.
i think in google equation there must some luck element but maybe one good PR5 link a about 5-10xPR4 can does the job fast?
Focus on building high PR Link Wheels (not quantity). Most of those backlink services are simple spam scrapebox posts which will get your site penalized by Google.
as a lot of people have said it all depends on the quality i.e PR and keyword relevance. Google algorithms take lots of factors into consideration links, traffic etc. (THIS IS A GUIDE TO EXPLAIN (NOT FACT)) - while it may take you 5x PR2 backlinks to reach PR1 - 50x PR2 to reach PR2 - 300x PR2 backlinks for PR3 - 1000x PR2 backlinks for PR4 - 5000x PR2 backlinks for PR5 the difference between PR0 and PR1 may be small but the difference between PR3 to PR4 is great. 100x PR1 links may be worth 10x PR2 or 1x PR3 backlink. so to clarify it is quite impossible to answer but a good start would be high PR, Relevant, Low OBL Backlinks and you should see it move up rapidly. hope this helps.
Lol all the answers to this are great, it takes one strong PR4 or PR5 backlink to pick up a PR4. PR is not a metric that will help you to rank though, so ensure that you are building natural links through quality content too.
You need High Quality Backlink to get PR4. The backlinks must from different IP to rank your site. Today google loves contextual links not blogrolls.