What Henry said. Plus, there's every reason to think that the Prius ('04 or otherwise) WOULD function as well and probably better than a regular ICE vehicle. There's no starter to worry about, minimal 'juice' is needed from the battery, the electric motor spins the engine up to speed with the large NiMH battery which we know works well to very cold temps.
The heater works better than most vehicles. The chassis is lubed like any other. They sell these things in Anchorage, Alaska--there's at least one guy on the Yahoo-2004 forum from there, they're experiencing single digit temps right now and he hasn't complained. Johh1701A's experience is well documented with his Classic and the electric motor is more powerful as is the NiMH battery in the '04.
Now, that isn't experimental proof I grant, but if one were to make a reasoned assumption as to the ability for the Prius to operate in cold weather I think it would be a safe bet to say it's going to be exemplary.
--evan