You're right, I didn't see that 4th prong.
That 4th prong is in a location that looks like it is consistent with the C15 "high temperature" plug. The high temp socket (C16) has an extra keying feature right where that 4th "prong" is to prevent use with a standard temp (C13) cable. The high-temp (C15) cable then has a cut-out keyway to mate with this extra keying feature. The high temp cable can also be used on a standard temp socket (C14) as there's no key to get in the way. That key is usually just an extra hunk of plastic, but it looks consistent with your extra "prong", here. It's probably electrically neutral.
http://www.bizsyscon.com/product/VARIOUS__POWER%20CORD%20IEC-60320%20C15__3715.html is an example of a mating cable with a standard US NEMA 5-15 plug. You can also get them (seemingly more readily) with the standard EU and British connectors. They're commonly used as "kettle cords" over there.
I'd replace the module with a more common C14 entry type. You'll be able to use any old "IEC cable", then, and this is not a high temperature application.