Why do you find that unbelievable?
I read back a while ago, that Iraq was buying parts for thousands of centrifuges -- apparently they can make a lot of smaller centrifuges, which are easier to hide.
I would assume that they would need industrial size centrifuges to procees nuke material. That number seems unbelievable to me. I wonder how many we have in the US? I bet not that many. Of course if they counted medical centrifuges like your hospital or dentist has, then 4000 might be believable. It just sounds to me like the gross exaggeration that might come from a place like North Korea.