I think 4 or 64 backlinks are not enough for any considerations. Try building lots of backlinks, thousands of them, and then you will see a increase in PR
I totally agree, it's the quality of the links that matters. I've seen a site with 20-odd links but one is PR6 get PR2 in an update before...
Quality definitely as important as quality. One backlink from a PR6 site will do a touch more good than one from a PR1 site, but lots and lots of PR1 backlinks will eventually be as good!