Theres not many differences between them. according to the website:
Fluval
04 VS 05 Series
Question What is the difference between the 404 and the 405?
Answer There have been so many design changes to the 404 while it was on the market that it is difficult to compare the various models with the
05 series. For this reason, the differences mentioned are from the latest
04 series versus the new
05 models.
When comparing the 405 to the latest version of the 404, they are very similar.
Aquarium capacity, flow rate and filtration volume are all the same.
The 404 is gray and blue while the 405 is black and silver.
The brackets that are used to hold the hoses over the rim of the aquarium on the 405 are more user friendly. They do not have the rubber bands like on the 404.
The 405 also has a larger, clog-free intake strainer.
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not worth the extra money, I think.