If my site is PR0 while the other site is PR4. I am quite new to this business and I suppose this is the right place to ask.
1 backlink alone will not be worth much at all but if you continue to build backlinks the higher valued ones will have have more affect on your pr than backlinks with little to zero pr value.
One "do-follow" backlink from a very high Pr site like PR 7-8-9 will be good enough to give you a PR but if you are looking for Pr from just one PR 4 backlink then i think it is not enough. You should have lots of PR 4 bakclinks to get any PR.
1 PR4 backlink in same niche might bring your website to PR1 theoritically. Here is the convert table for reference: http://blogbloging.blogspot.com/2008/06/how-many-backlink-needed.html