What size CFM inline fan?

I had a question hoping someone would be kind enough to help me out.

I'm going to be buying a 4x4 grow tent from ebay and also a 4" 170cfm inline fan and carbon filter combo from ebay the company that sells it is HTG. Anyway will this 170cfm fan be enough for my 4x4 grow tent?

I have a 400watt HPS, no cool tube or cool reflector.

I will be growing around 8 plants.

Input much appreciated, thank you

What is the temperature outside the tent, err where you are drawing your fresh air from?

Sorry, Temps outside the tent are usually around 60-70 it ranges throughout the night and day never past 75 though.
And I will be using a passive intake.

the 4" fan would probably do the trick. A 6" would give you some cushion, just in case you ever need it.

Thanks for the input I will consider a 6" fan