If I will buy 4 PR7 backlinks will I also get a PR7 ?! The PR7 pages&websites are similar to me and they only got 10 outgoing links / PR7 page
You won't get PR7 and I doubt you will get PR6. Google may devalue the links if its spots they are paid links and you might get PR0.
I don't think google will penalize for these links, you'll get 5 or 6 PR, depending on ammount of links on those pages, because PR will be distributed between all links.