I agree with NickCoons - it doesn't matter if the links are from different domains or not. The fact that bestBBS.com has only PR4 can be explained if you are looking at the PR of the linking sites and the numbers of links. The site which contributes most is www.webmasterworld.com/home.htm. This site has PR7. However, there are more than 100 links on this site which leads (estimated) to PR4.