Personally I would respond like this (I am not telling you how to respond, just stating what I would do)
I would respond with a positive first.
Thank you very much for your offer, it is an offer that is definitely in range of my target selling price.
Then the not so negative
I was thinking more in the $5000 range for this one but if you would like to make the deal fairly fast I could be persuaded to meet you half way for a total selling price of $4000 USD.
This way the buyer can deal and you are not accepting too fast. It is close enough that they probably won't walk and it allows them to realize a discount. The response for me is pretty standard and has been very effective for me when closing deals.