HI bud,
That sounds great for ventilation. If you go with 8" ventilation, ideally you'd want an 8" cool tube or air cooled hood too but I know yer tight on funds... All in due time. With a 4" flange on your hood it's going to really limit the pulling power of even an 8" fan. It's going to take some serious air movement to keep things cool and that sounds great to me. You should have some time til the blast furnace kicks in with outdoor temps but if you can manage to get great ventilation going you'll be on your way to having great control over temps in your grow space. I dont know if youre familiar with fan motor speed controllers but they are very handy and made for controlling the speed of your ventilation fan. There is one controller that is pretty affordable ($22) called the Speedster with a dial for adjustment.
h
XXp://www.horticulturesource.com/product_info.php?products_id=1665
I have one of those but recently I bought a new thermostatically controlled fan motor speed controller that is more pricey but to me it's well worth it. It's made by Grozone and the model is a TV-2.
h
XXp://www.grozonecontrol.com/TV2_en.html
Anyhooo... keep on rockin.
Peace!