Charcoal Elf Filter

Does anybody see a problem with using the Elf, which has a 4 inch intake diameter, putting a 4'' to 6'' reducer on it then have the 6'' ducting go from the reducer to a sealed light then to ducting and then out via an inline fan?

I am concerned that the 4 inch filter into the 6 inch duct might cause airflow problems. Thanks.
 
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