The 04 series is now superseeded by the 05 series. The 04's aren't made anymore I don't think, as the new 05's have replaced them. Whats the difference? Colours on the cases is the only obvious one
The specs and innerds aren't that much different IMO.
The 04 IMO were rubbish, leaking, loosing flow and becomming noisy very quickly in my case and in the cases of a few friends of mine tht have also had Fluval 04's. Mine was quite shocking, leteraly
(moter head leaked voltage into the tank, administering electric shocks to anyone that placesd their hands into the tank without switching the filter off first)
The Fluval was my second exturnal, and when I bought it, is was the best thing since sliced bread. Now I hate it after the cosnstant fiddling with it to keep the flow up and the relative aquardness of working on after the brand new priming pump stopped working again after the second use... Fluvals are't too hard to work on, but realtive to other makes, they are a royal PITA. Reliability is questionable to say the least also...
The 05 are banded about as very good. Will they stand the test of time? I sincerly doubt it. The 04's were good when they first hit the market, and not much has changed on the 05's TBH IMO
You'd be better looking at Aqua One and/or Rena on that price bracket, if the relative costs of the three manufacturers in the US is the same as near me
If you want the best filter available in your area, check out the Eheim Pro 2's
All the best
Rabbut