Take a look in the Lighting FAQ. Its in the "Hall of Fame" sticky and in my sig.
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At one point I made my own conversion housings to fit a H7 bulb into a 4703 housing. Here is a thread where I talk about installing H7 bulbs into 4703 housings:
clear head lamps for trans am
(Scroll down to my post #9 in that thread for an outline, since the original site changed and the link is no longer valid.)
H7 is a low beam only bulb instead of the H4's low/hi, but the H7 is brighter (it puts out more lumens than the H4's high filament).
Unfortunately these bulbs weren't meant to have their backside exposed like that (neither is the H4) and it eventually filled with water and a loose connection melted the stock connector. If I covered more of the back of the completed modification with clear silicone to seal it (read my post in the other thread for clarification of how its sealed) maybe it would have done a better job of keeping the water out, but the added thickness of the sealant probably would have made a worse connection to the harness since the pins are relatively short. That could be solved by cutting off the stock connector and making a sealed connection to the back of the bulb and a new custom connection to the car. Unfortunately, then it wouldn't be a quick/simple plug-and-play to install new bulbs later if one blew. Also, this is all a maybe since new, unmodified, 4703 bulbs eventually fill with water anyway,
Here is what I am talking about with respect to the custom connection:
That was an attempt I made by installing the high-beam 4701's bulb into the 4703 housing. The beam pattern was somewhat close, but way too much light went upward into peoples cars.
Here is what my stock (melted) 4703 connector looked like when I cut it off:
I now have Hella 90mm H9 projectors.
http://www.ls1tech.com/forums/showpo...91&postcount=6