I would start off by offering half (as a maximum, less if you can) and sticking to the offer. The thing to remember is that they may have acquired the domain for registration price so even $2000 will be a large profit, and they may not have another buyer for years.
If that goes nowhere, you can always go back later and pay the full price or whatever they have come down to.
IIRC, they also sell domains which they don't own, and have less leeway for negotiation on those.
I would start off by offering half (as a maximum, less if you can) and sticking to the offer. The thing to remember is that they may have acquired the domain for registration price so even $2000 will be a large profit, and they may not have another buyer for years.
If that goes nowhere, you can always go back later and pay the full price or whatever they have come down to.
IIRC, they also sell domains which they don't own, and have less leeway for negotiation on those.