64 would also be marked 4. It's pretty easy to tell the difference between the two.
If they ever made a 44cm - which sounds like a terrible idea in a 700c - I'm pretty sure they'd label it a 4 as well.
Unfortunately, I have no idea if the Meise factory is still open. They're listing a different address on their website now, so it's possible.
The correct address is merckx.serials, and the domain is .org, not .net, just in case you have any additional q's or anyone ends up on this thread looking for a way to track me down.

Unfortunately, I have no idea if the Meise factory is still open. They're listing a different address on their website now, so it's possible.
The correct address is merckx.serials, and the domain is .org, not .net, just in case you have any additional q's or anyone ends up on this thread looking for a way to track me down.