I am probably going to move this to Law questions but I can give you information based ony my personal expereinces.
As far as the website registration and access are concerned, who has control over that?
You would have a domain host like Godaddy, Network Solutions etc.. From these webstes you can set information like the website owner (owenr registration) the nameservers (pretty much points to your website) and other settings.
If you never once asked for your website to be moved from one server to another then that's this guy's first mistke. He literally TOOK your website, and now when you try to dispute an outrageous bill he is holding it hostage.
I would first demand that he return the website to it's original host and also return it to it's condition before he made any changes.
I would then ask for a accurate and detailed bill as so WHY he want's $4,000 (cost breakdown)
Then, in writing you should notify him as to why you think $4,000 is outrageous and that you are refusing his work.
Website design and implementation is all about what the client wants. I can say "I want $5,000 to build your website" But if it's not what you wanted, or when I finish I say "oh by the way it's $10,000" you have a right to refuse it.
If the guy spent weeks on it and had to make significant changes etc... lets say he spent 100 hours on it, then yeah maybe for his expertise it's worth $4,000... but honestly he would of had to build you an entire new website... because that's one hell of a price tag for someone just adding something.
I suggest you seek legal counsel, what is for sure is that you never once authrized him to move the website, he did it, and now he is holding your website ransom.
You trusted him with private information that allowed him to maliciously move the website and he also caused it to cease working once you disputed his bill.
Get a full invoice from his business.... If he doesnt run a business or if he's not invoiceing you, let him now of the repercussions of not reporting income or not charging taxes.
Best of luck!