its me again, i stopped at work (napa) on my day off to check some things out. earlier in another post someone asked about h4s being put in. but the bulb in the scion is a 9003. checking them out at work theyre the EXACT same bulb..................... O.o i dont understand it either but i asked one of my coworker countermen about it and he said the H4 might be a brighter bulb all together. since its just an auxilary bulb. like for offroad trucks with KC lights or something. im not sure if its true or not but it might be. something for you all to ask other mechanics/parts specialists about. reply on this thread, give some others your knowledge

If you want to be 100% sure that you're getting a legal bulb with the elements in the right position, you need to get the 9003. I went with Sylvania SilverStars 9003ST and I love them. The H4 bulb is shaped the same and it will fit, but I don't know about element placement or beam pattern.

My understanding is that the 9003 is like an American version of the H4. The DOT specs are more specific about the exact location of the filament in the bulb.

basically the same. only time it matters is where it will state one or the other. some housings have reflectoprs for a certain bulb. so on and so forth

I used h4 piaa xtreeme whites. 2 months later I had a problem with my relay, the mechanic told me that the wattage was too high and that i should get 9003 which will have a correct beam pattern. So now Im rollin with sylvania silverstars. Love em better than the piaas.