All Gen 4's are the same in power needs....my 604wifi is indeed 5 volts and the charger is rated at 2.0Amps. (By the way, I also use 1.6 amp chargers from my older Archos AV420's which also work just fine.)
The "central hole" of the plug is positive and the exterior of the plug is negative (according to the diagram on the back of Archos charger's transformer/main body bit).
So you now have all you need to buy/find a charger!....and yes I agree that the Travel Adapter is a very expensive item not coming with it included. For the novices reading, the mains AC charger only comes with Docking Station and DVR Station (but will plug into all three.)
I think they call it "Marketing"...I know what I call it!