My site is have PR4 but the backlinks from google is very small.What are the best methods to get backlinks from google ?
are you takling about indexed pages? then get more content and links in your site if backlinks you mean by searching link:yoursite.com - then just buy links on other sites, note that this backlink updates once every few months unlike the indexed pages
Check this blog post - http://www.seminc.com/blog/google/ask-sem-inc-how-to-get-more-backlinks-from-google/
There are MANY techniques for building backlinks. You can purchase them (though G is actively against this), trade links, submit articles, submit to directories (G will only pick up higher quality directories), comment on other blogs (not spam), and more. I'm working on a link building book now. Feel free to PM me and I'll send you the URL where you can sign up to be emailed when it is released and get it at a reduced rate. Best of luck!