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I just ordered this internal filter for my 4g: http://www.thatpetplace.com/ovation-210-submersible-filter
It says up to 15 gal but it's only 53 gph.
I got that one because I am trying to fit it into the sump of an aqueon evolve4, but if you have a normal tank you might like a HOB better. http://www.truaqua.com/aquarium-hang-on-filter-pfe1.html
Here's a HOB style that's 45 gph.
By the way, WCMMs need a much bigger tank than 4g. They need a 10-20 gallon tank.
I agree, for the price its hard to beat. But if you want to splurge and have the option of using it on a bigger tank (up to 20g) a aquaclear 20 would work. They are about $20
The smallest aqueon quiet flow has a flow rate of 100 gph.. That seems a little too strong. And the Azoo palm is only 16 gph which is very weak. For a 4g you want something in the 30-50 gph range. But I suppose it depends on what kind of fish you want to keep also. For example a beta would not like a strong filter.
WCMMs need a much larger tank. They are very fast active swimmers.
WCMMs wouldn't mind a high flow but shrimp would probably prefer a lower flow so they don't get blown around.