My facebook fanpage's PR is 4. I'm getting backlinks from that page. I want to know is those backlinks are really helpful or not. and those are dofollow or nofollow?
You can check your self that if your links is dofollow or not by looking at source code. Anyways, if the link is dofollow it will help you gain more PR juice, if not than it may help you get higher SERP rankings.
If you want to check if the FB links (or any links for that matter) are Dofollow or Nofollow then you can check it yourself by viewing the page source of the page and checking from there.