will a 4" inline fan with duct reducers on both sides be enough to cool down a 400 watt hps in a cooltube? will also be using a carbon scrubber to cover up odor with same fan (if possible)?
Seems 150cfm/tube minimum, but since you want to install the filter into the same duct run, you'll need more 'cfm' because of restriction and consider how many ft of ducting you wil use. Often it's better to go with a larger fan, such as a 6" that blows around 400-450cfm with a speed reducer. You can minimize noise from the fan and know that you'll have plenty of air [and stink] being removed, instead of running a 4" full blast and perhaps still dealing with heat (odor) issues. If you have the space, go bigger, it's worth the extra $$$.