4000 BC would be an approximation of the time of Joseph.
Moses was in the time of the Thutmose Pharaoh's (ca 1400 BC) + Joseph was about 400 years before that. That gets us back about 3800 years ago. Curious,isn't it?
It wouldn't be 4000 BC but 4000 years ago. Eygpt was not a high culture in 4000 BC and the Great Pyramid is allegedly about 4000 yrs old.