@Mauler
They should have received a pre-release CD about three(?) years ago. You can install OS4 from there, but it's probably better to register at
http://www.hyperion-entertainment.biz with the serial number on the back of the case, and download "OS4 final update".
If you don't have the CD, first port of call will be the dealer, second email Hyperion with the AmigaOne serial number, dealer name and any other information and maybe they can get you access. It might be that this board didn't qualify for an included OS4, in which case you will need to part with some cash.
Hope that helps.
Chris