I've been playing around a bit more with the UK eLED module. I haven't taken it apart (yet) or done any real voltage/current measurements, but I'll bet it only runs fully regulated on high power mode in the C8 eLED light, and not in the C4 eLED.
Both lights are pretty bright, but the C8 eLED is noticeably brighter on high, and in a regulated light this should not happen. The regulator is supposed to compensate for variable battery power and supply constant power to the light engine. On low mode both lights seem equal, but on high there is a difference. If it was a proper buck-boost regulator then light output would be the same (at the expense of some additional battery life in the C4).
Since this doesn't seem to be the case, I suspect it is a buck regulator only, and the C4 is running in direct-drive mode on high. A buck regulator needs to have a bit more power than the device it is trying to drive. For instance, if you are trying to power an LED with a vf (input voltage) of 6.8v, the buck regulator would need more than 6.8v to function. Let's say the regulator needs 0.2v over vf to run; that means once the voltage has dropped below 7v the regulator will no longer funtion. Most buck regulators will go into "Direct-Drive" mode once vf is too low. Direct Drive means it is just passing all of the power through (minus circuit resistance losses).
If this is the case (and I'll bet it is) then the C4 is only running off of 6V (minus circuit resistance losses) on high, and the typical Luxeon 5 watter needs about 6.8 to run at the correct current. This explains the brighness difference, and also why the C4 dims more as batteries are used up.
Aloha, iG