As long as you are looking for traffic or serps the PR4 site would be much more beneficial. If you are solely looking to increase your PR then the PR6 site may help you more.
You have done a good job. Go for PR4. They will help you in acheiving what you desire. For better results, from my point of view i will suggest to go for a site of PR 6.
Your site will benefit from your both links. But it has much weight on the PR4 site because of the target links and the quality of site (this will help you to improve your SERP rankings and targetted traffic)