I am simply using Adwords CPC to set a value on these 4000 visitors
FWIW, AdWords advertisers who know what they're doing are unlikely to pay the CPC that Google suggests.
You're right that CPC prices can be a useful starting point for valuing a domain, but potential purchasers would crunch their numbers based on what they really pay, not on what Google suggests. Expect that to be a lot less than you're envisioning.
In addition, potential purchasers would also discount what they'd be willing to pay to allow for the risk that traffic from the search engines can be seriously unstable. Past performance is no guarantee of future performance.