H4 Bulb
A dual-filament headlight bulb that handles both low beam and high beam in a single unit. Common in motorcycles, older cars, and many Asian and European vehicles.
H4 is unique among headlight bulbs because it contains two filaments in one bulb — one for low beam and one for high beam. When you switch between dipped and full beam, the bulb activates a different filament at a different position inside the reflector, producing two distinct beam patterns from a single socket.
This dual-filament design made H4 the default headlight bulb for decades. It's still standard on most motorcycles (which only have one headlight), many trucks, and a wide range of Asian and European cars. In the US market, the same bulb is often labeled 9003 or HB2 — they're all the same thing.
LED H4 replacements are among the trickiest to get right because they need to replicate two filament positions accurately. The low beam chips need to sit where the low filament was, and the high beam chips where the high filament was. If either is off by even a millimeter, the reflector produces the wrong beam pattern. Quality LED H4 bulbs use a copper or aluminum heat pipe design that positions chips precisely, but cheap ones rarely get it right.
Specifications
| Type | Dual filament (low + high beam) |
| Standard wattage | 60/55W (high/low) |
| Also known as | 9003, HB2 |
| Common in | Motorcycles, trucks, older cars |
Related Terms
- H7 Bulb
A single-filament halogen headlight bulb commonly used for low beam or high beam in European vehicles. One of the most popular bulbs for LED headlight upgrades.
- H1 Bulb
A compact single-filament halogen bulb used in headlights and fog lights. Smaller than H7 with a different connector — common in older European and Japanese vehicles.
- Projector Housing
A headlight type that uses a convex lens to focus and shape the light beam into a sharp, defined pattern. Handles LED and HID upgrades much better than reflector housings.
- Reflector Housing
A headlight type that uses a chrome-coated bowl to bounce light forward. Simpler and cheaper than projectors, but LED upgrades can cause scattered light and glare issues.
