9004 bulb size is also referred to as HB1. Commonly this size is used for the headlights and it is a dual beam. There are three Xenon options for it when upgrading the lights. It can be Bi-Xenon, Low Xenon/High Halogen, and Single beam kits. First two types are dual beam kits which perform for high and low beams (2002 Dodge Ram 2500 and 2000-2001 Nissan Xterra). The difference between them is the type of the high beams light. With Bi-Xenon set you will have all beams Xenon(HIDs) while with the Low Xenon/High Halogen kit only the low beams will be Xenon(HID), while the high beam is just like your factory halogen bulbs. 9004 Single beam bulbs are used predominantly for low beam conversions.
If none of your lights come on, only one beam is working, or your high and low beams are reversed, you can try repinning the OEM connector in order to change 9004 kit to 9007.
9004 (HB1) size is also available in LED version. Kensun offers you two options: LEDs with Cree and ETI Chips. Bulbs with Cree chips have removable locking rings so you can install it into the housing at first and then lock the bulb in.
LED bulbs with ETI Chips are designed in different ways. They have a separate driver to provide the separate power source for the cooling fan and the Chips themselves.