What is the real difference? I mean I know(NOW) that they are the same bulb type. But they have different names so there must be some differnce. Are H4's normally an aftermarket wattage? Are H4's like a metric size of lightbulbs?

 

H4 = 9003... They just have diff. names but are all the same bulbs.

 

Supposedly, 9003s are built to tighter tolerances than H4s. Or vice versa. Performance-wise, you probably won't notice it.

The ANSI #9003 is a DOT designation for the bulb that an HB2 housing uses

H4 is an SAE/ECE term for that bulb.

I wouldn't worry about the nomenclature...just remember that it is the best dual-filament bulb on the market. An overwattage bulb (not 55/65), whether refered to as 9003 or H4 is not DOT or ECE approved for use on road. Also, I would not use an overwattage bulb without using a special harness with beefier connectors, wiring and higher amperage relays.