How do I cool a reflector w/ 6" flanges?

I have a 180 cfm hydrofarm with only 4" flanges, and a reflector (sunleaves) has 6" flanges. How do I make this work?

With this: http://www.htgsupply.com/viewproduct.asp?productID=53753

Or go to your local home depot an pick it up

With a 6" Vortex fan or similar.

Edit: sorry, I see that you already have a 4" fan. I have run into the same problem. Just
go to Home Depot or Lowe's and buy some ducting, and a couple of 6"-4" reducers. They
have two ends, one is 6" and of course the other is 4".