When I was being raised as a young earth creationist, the earth was supposedly 12,000-20,000 years old. Then it was 10,000 years old. Then only 6,000. After I outgrew that nonsense, I joked that in a few decades YECs would say that their god created the earth in 1980, and anyone older than 40 are agents of the devil sent to test your faith.
6,000 to 12,000 years old is what I heard. I'm guessing that this "Christians Against Science" page is a joke community that is making fun of YECs by saying it's 4,000.