Assuming your payoff for that 4k is 3 months ignoring energy costs...
600 * 1000 * 60 * 60 * 24 * 30 * 3 / 4000 = 1166400000 thousand hashes per dollar. If the average person spent 60 at 20,000 Hashes seconds on your site you would need 972,000 people to connect to make a dollar... ouch.
