4 inch Inline fans are generally good for 600w HID systems or smaller. Use fans to keep your grow room at the right temperature & humidity, to ward away pests, and to keep your plants strong. A gentle breeze over your plants from an oscillating fan can help stiffen your plants main stalk, making it better at holding its flowers and fruits without falling over. Bugs also hate wind, so keep that fan moving over the leaves to keep pests from setting up shop. Don't know what CFM (cubic feet per minute) inline fan you need to exhaust your HPS or MH light? Read our guide on how to find the right CFM fan for your indoor grow room.