9004 bulb size is also referred to as HB1. There are three options for this size when upgrading your car lights. It can be Bi-Xenon, Lo/Hi Halogen and Single beam bulbs. First two types of bulbs have a dual filament meaning they will convert both High and Low beams. The main difference is in the High beam: with Bi-Xenon you will have both beams Xenon and with Lo/Hi Halogen the High beam will be a regular yellowish Halogen light. 9004 Single beam bulbs are used predominantly for converting Low beams.To make sure that this is the correct bulb size for your car lights, you can look it up on the bulb itself or in your car owner’s manual.