At the beginning of this post, I was thinking 4k was a good price. But I see the .com is taken by a blog with no new posts in 2 years...and an about page with links to a variety of crypto people including the COO of Coinbase and the Winklevoss twins. I didn't check to see if the content was actually original or not. If it's not then it could be this site is going to be listed for sale eventually.
The .org and .co redirects to the .net which is a page showing a guy with the last name Nakamoto's tweets. So he's probably not looking to give those up for 4k.
I wouldn't want to own Google.net and try to actually build a real brand while Google owned the .com. But I could see competing with a parked blog and some tweets.
I think 4k is a good price for it. But I think there's potential to get more from this buyer.
You mentioned you thought the buyers offer was firm. If you have it priced at 99k and they offered 4k firm, then I think any sort of counter will probably be declined. So If I was going to negotiate, I would probably decline and assuming DAN lets you include a message, I would probably say something like "Thank you for the offer. As you said it was firm, I won't waste your time by countering and unfortunately I am not willing to part with the domain at that price."
That provides them the chance to either up their offer or more likely ask you to negotiate against yourself by lowering your price.
I have only negotiated half a dozen times on domains (including buying and selling) and no negotations on sales this high so I might not be giving the best advice.
