9004 is a dual filament bulb. It has a low beam and high beam built into the same bulb. When choosing your HID kit you will need to determine if you want HID lights on just your low beam or on your low beams and high beams. If you elect to for both than you will need to upgrade to the Bi-Xenon HID Kit. Included in your purchase of this HID Kit is a pair of premium ballasts and a pair of premium Xenon HID bulbs along with necessary installation accessories. 

9004 HID Conversion Kit Installation Manuals:

9004 HID Conversion Kit 

9004 HID Conversion Kit (with Relay)

9004 HID Conversion Kit (with Cancellers)

9004 HID Conversion Kit (With Relay + Canceller Combo)