Keep one thing in mind, guys: an H4 bulb is 55w/60w (sometimes 55w/65w). This means that regardless of how you derive the power to light it, it will require 55w/60w. Adding HID does not change this wattage, and HID does not make extra power: rather, it uses MORE power to create the hi-frequency bulb power. Be careful what you pick out for this project!
At one time, there were halogen bulbs of 35w/45w power. The light, like typical halogen, was far brighter. There was an entire retrofit from Rocky for the 350F in 1974, when I was still a racing wrench. It was $35 (1970s money), very expensive at the time. I haven't found those H(?) bulbs yet, but when I do, I'm going to present a kit for the Little Fours...
It's possible, I hope, to install a single-beam small halogen in the 162mm size, thanks to Candlepower. But, they don't seem to have ALL the parts, still working on it....even a 40w halogen would be a big improvement!