Speed controllers don't work well on the 4" fans (creates a loud hum - though it doesn't hurt the motor much). They work great on the 6" and up models... the standard one that is out now works great (it is actually a router controller and can be found at many hardware stores for about $15 - though it may be special order. Hydrofarm, Sunlight Supply and the others have all adopted it as a fan speed controller.
http://www.hydrofarm.com/pb_detail.php?itemid=8589
The 6" with speed control gives you the most flexibility - with plenty of overhead for the hot days.
