Instead of replacing your fan, filter, ducting, while downgrading to a 4", you should pick up a fan speed controller fo $15 tops. It will allow you to adjust the fan speed which is good for two reasons. Regulating temperatures as the season change, as well as being able to keep the sound level down (if stealth is any concern). A dimmed 6" is super quiet,
Good luck
great idea skatter...speed controller ordered! as well as cal-mag that namvet suggested! ready to go next week
