There are HID kits out there which generally use a D2S bulb and a base machined to fit in a halogen bulb socket. The thing with H4 is that it's a dual filament halogen bulb (i.e. low and high beam in one bulb) and that's a bit trickier to pull off with an HID bulb.

HID bulbs are classified by color temperature. Lower numbers like 4100k are more like blueish white, and the higher you go the more purple it gets. 4100k is the best from a visibility standpoint.