I would not risk going with a 4" fan if you can avoid it. A lot of people would say 6" is over-kill, but I would disagree. You can put it on a fan controller and really cut down the volume as well. Because they are loud, such as any in line fan is. And then if it gets really out of control hot, instead of your plants being in dangerously high temperatures, you can easily crank it all the way up and get some serious air moving to save them.
Not to mention, if you ever want to upgrade to a 600 watter you already have a fan that can handle the heat.
I'm new here and I completed my first grow using a Homebox XS with 4 Blue Ryders. I'm going to be buying the Secret Jardin DR100L in a few days. I'm going to run it with 400w MH/HPS and I'll be growing autoflowers due to time constraints.
I'm not sure if this setup would work, but I was planning on using my 6" 435cfm fan to cool my 4" reflector using the 2 ports provided for reflector cooling. Then I would suck the air from outside straight across the reflector via ducting and out the other reflector cooling port. Hope this makes sense thus far.
Then for odor control, I would use a 4" 171cfm fan for exhaust and run ducting to the carbon filter hanging inside the tent. And I was hoping that I could run this on a timer. So 5 mins on, 15 off for example.
I'm open to suggestions and opinions. Thanks.