Barely informed guess: M5.
If you've got a hardware store, take it on down. I've got a great local hardware store; they love the challenge of finding the right size bolt, and can usually find one within 5 minutes. Then they charge me what's listed, 17 cents? each.
Stay away from the big box stores (Home Depot, for example). Buy a packet of 6 screws for $1.95 and try it. If that doesn't work, buy something else.
Fastenal splits the difference IME. Some are almost as good as the hardware store (if not better, they have a bigger selection), and some are overgrown HD hardware departments.