I've had the
www.hidkits.com Bi-Xenon H4 kit on my car since August. It replaced an SPW Bi-Xenon kit that I bought in June, and hated. (The SPW kit I bought had the 7500K color temperature bulbs, which were annoyingly blue. Marginal illumination compared to quality halogen bulbs, and a lame-*** high beam.)
The hidkits.com kit is excellent. Check out the animated GIFs on their web site to see what they do - the H4 kit uses a custom bulb base with a solenoid that actually repositions the envelope to provide low and high beam. So the arc is in the correct focal position for both low and high.
I should mention that my car (a '97 Probe GTS) came stock with those nasty H4666 sealed beams. I bought Bosch H4 headlamps from Dan Stern 4 years ago to replace those things, and the improvement was definitely dramatic. The HID kit I have now just mounts in my Bosch lamps and works very well.
I've had many long conversations with Dan, and while he's obviously devoted a lot of his life to the specifics of the automotive lighting industry, I think he's a bit more of a stickler than necessary.
H4 bulbs use axial filaments, and HID bulbs have an axial arc. In this respect, an HID conversion is not as terrible as it might be for some other 9000-series bulbs with transverse filaments. It is definitely impossible to attach an adapter ring to a standard D2R or D2S HID bulb and stick it into an H4 lamp and get a proper beam pattern; the arc will be nowhere near where the low beam filament was. The only way to put the arc where it needs to be is with a custom bulb base. Both the SPW and HidKits conversions use custom bases for their Bi-Xenon kits, and they both produce an excellent E-code low beam.
Both of these kits use a solenoid molded into the bulb base to toggle hi/lo beam. The SPW kit mounts a movable shield, that is pulled out of the way to produce a high beam. The resulting high beam is pathetic, it doesn't actually put any more light on the road. The result is less than optimal because the arc itself is still sitting in the low beam's focal point, and an H4 lamp is only designed to produce a low beam pattern from that point source.
Since the HidKits base actually moves the entire HID capsule, putting the arc into the correct focal point, you get a decent high beam as well as low beam. I love it.
I had a web page with my installation notes and pix but it seems to have gone offline. I may try to re-upload it somewhere else in a few days.
There's a limitation with this kit, which hasn't been a problem for me yet - the solenoid takes time to recharge after you trigger it. When you go from low to high, if you drop back to low, it will take 2-3 seconds before you can go to high again. So you can't flash quickly low-high-low-high. Since I seldom flash people, it hasn't been an issue. And otherwise, the transition low-high or high-low is instantaneous.
A lot of people I know with H4s have been looking into replacing them with round projector lights. I thought about it too, but this to me is a far better approach, and far more manageable. I can barely find space for one ballast on each side of the car, if I had to use two separate HIDs for high and low on each side, installation would be impossible.