First, just to be sure that we're all singing out of the same hymnal,
here's a link to a description of the Fluval 4:
Assuming that you're following the recommendations about tank size
(50+ gallon) for a bimac it would appear that you'll need at least two
of these to provide sufficient filtration. Keep in mind that an
octopus is a messy eater with a high metabolism and a big appetite.
The consensus seems to be that you need two to three times the
filtration capacity for an octopus than you would need for an
equivalent amount of fish. The Fluval 4, which seems to be the largest
of this type of filter available, is rated for a 57 gallon tank which
means that two of them might be enough but one definitely wouldn't be.
One other minor point. The pictures that I can find don't show
sufficient detail to tell much about the intake screen. Octos have a
reputation for injuring themselves on pump impellers so you might need
to install some sort of auxiliary screen to keep curious arms out of
harms way. Then again, the existing screen might be OK as is.
You might want to do a Google newsgroups search on "Fluval 4". I just
took a look and there seems to have been quite a lot of activity (530
hits) about this filter so you ought to be able to learn something
about its effectiveness and reliability.