If you really want to make something slick, try your hand at a HID arc flashlight. Those are the spiffy new ones coming on the top-of-the-line luxury cars. Not only do they run on low voltages (because they need a step-up circuit anyway) and are obviously VERY bright, but they also happen to be very efficient! A set of D cells running one of those lights would likely last several hours. Lumens per watt is way more than Luxeon.
A halogen car low beam is usually about 55 Watts/12V (H4, H3 etc). That results in a current of more than 4 Amps (!), and I dont think 4 CR123s is able to give you 12V at that current. You probably have to use 5 CR123s or connect 8 ones using a parallell/serial configuration to get high enough voltage.
For comparison: The Surefire 500 lm N62 lamp is using 4 CR123s with a current of 3.7 amps. At that current the batt voltage is below 8V. (Source: Brocks flashlight site - look at the top of the table at the bottom of the page).