Adam and 4000 BC

4000 BC is the accepted date for the creation of Adam, or so I've heard and read from numerous sources. Though I've also seen 4004 BC touted by some well known bishop that did some calculations or something.

I want to know if anyone here can tell me how this date (4000 BC) was acquired, or at least point me in the right direction. I don't mind researching myself but my google searching has been mostly fruitless for the purpose of answering my question precisely.

Thanks guys