Ok, that makes things rather more clear, you bought an existing domain that had existing PR.
A whois search for acmepanel.com shows a current record from Jan 2010 so they had that domain name already before they sold the .biz.
I would think it very possible that the domains were linked to each other so that PR passed from .com to .biz (or the other way round) but that these links have now been removed. Also one would imagine that any other external links that were pointed at the .biz domain would have been transferred by acmepanel to the .com site. (Interestingly though, the .com is PR4 now as well, so if there was a 7 it has been lost altogether.)
My understanding of the value of an aged domain is the established links that come with it, so I am not sure how that fits in with your scenario as you said that it now only had one link.
But as you have now explained that you know the history of the domain, I am a little confused as to why you started the other thread asking how it could have PR4?