Logic Puzzle: "Rock On"

[The Armchair]

Before I leave work for the day:

You have of those counter-balance things and a big, 40-pound rock. You use the balance and the rock to measure out 40 pounds of hay, which he sells at the market.

Your annoying neighbor comes by and asks to borrow the rock. Begrudgingly, you agree, figuring not even he could mess up your 40 pound rock.

Bzzt.

He comes back a day later, hands in his face. He tells you that he broke your rock.

As you go for your hoe (the garden tool, you fool!), hoping to drive it into his back, he yells, "But wait! It's an improvement." A reasonable person, you hear him out.

"You now have four rocks. And I tested them on my super-accurate bathoom scale [which you've seen, and you trust], and it's really cool. With the four rocks, you can now find any weight on your counter-balance thingy, 1 to 40!"

He offers to let you weigh each of the rocks on his bathroom scale, to prove it to you. If he's telling the truth, what should each rock piece weigh?