Probably a bad idea. Those tend to lose PR by the next update. How will you 're-create' the old back-links which earned the PR4? That would take a copy of the original website (and database)
I wouldn't buy a dropped domain either, might as well start with a fresh one. If it made it all the way through expiring and then even dropping, chances are nobody saw any value in it at all.
Dropped domains tends to loose their pagerank. What's a non-confirmed pagerank? If its a fake pagerank, info for another domain when doing a info:domain.tld gogle search, then its worth is next to nothing.
I don't understand what you mean by this. Is this a way of checking PR? How does one check to see if the PR is fake?
Do a search on Google for info:domain.tld If Google returns no result, it's fake (I think). If Google returns a resut, but the link goes to another domain, it's fake. If Google returns a result, and the link goes to the domain you searched for, then the pagerank is good.