If it is from a relevant, authority source then it might have some effect; I don't think you should expect anything more than a PR2 though.
It depends on a lot of factors....how many links are on that page, is the actual page of your link pr4, how many other links you have, etc. At the most it could boost you up to PR2. To raise your PR much you really need more than just 1 or 2 higher PR links.
an authority source is a website trusted by Google, an old established website with a strong backlinks could be an authority site.
It's based on linking site/page if it's a strong PR4 with low/no external links except yours then a PR3/4 might be given, but internal links are also considered in the calculation.
Yes, on its own it won't do very much. But well done, keep working at it and get many more of them. You will reach PR4 if you get several more as well as other links such as forum sigs and good link exchange
I like how this thread was moved from the Google forum where every other topic is a PR question. Can you leave me alone "Shannon"