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Of course a link from a PR7 should help, but by how much would be impossible to say and to quantify as there are plenty of other factors involved.
Also note; It is widely assumed that Page Rank is actually held and calculated at a fractional level (to how many decimal places I don't think anybody outside of the 'plex knows), and therefore whilst you may well get a boost from the PR7 link, you may still only show as a PR4.0 to the outside world.