H4 bulbs are a type of halogen bulb. HID bulbs use a special xenon gas filled bulb connected to a ballast which generates a high voltage. The high voltage creates an arc in the bulb which creates that light. Because of their design they need a special projector to focus the light and control cutoff and glare. They do sell kits to put these bulbs in H4 or just about any bulb housing, but they really need the projector to get the most out of them. My brother has them in his Tiburon, and while they work okay they would produce far less glare to other drivers and light up a much greater distance if he had installed HID projectors. That's another thing, halogen bulb projectors are not the same as HID projectors and won't work right either. It's kind of like when you replace our incadescent bulbs with LED's, it's a totally different kind of light and doesn't glow in all directions like conventional bulbs. Another issue is low and high beam, most use a seperate bulb for each so you lose high if you dont use a bixenon setup. Most cheap aftermarket one's use a conventional halogen for high. OEM bixenon units use a servo motor to angle the bulb or an electromagnet to pull the arc up or down. The ones I will be using for my low beam positions are bixenon projectors out of an Infiniti FX35, they use a solenoid to tilt the bulb.
