as I understand it, the original design accommodated 2 18650s... and you are *thinking* that switching to a 4xRCR123 will open options for brighter lamps...
unfortunately, I don't think this is the case,... I'd stick with the 18650s, because a single 18650 tends to have about 4 fold the capacity of 1 RCR123, even though it only takes up approximately the space of 2 123 cells.
A RCR123 under high current demands will deliver about 0.5AH capacity, and be able to deliver about 1 amp safely, up to around 1.5A if you want to push it and reduce cycle life a lot. Around 22W at best from 4 cells.
A 18650 under high current demands often still holds up around 2AH capacity, and can safely deliver around 3-4 amps, up to around 5-6 amps if you want to push it and reduce cycle life a lot (with unprotected cells). That's around 40 watts of output potential in a "wow" factor light.
So.... with that in mind, I can't really recommend a bulb, but can suggest gearing your bulb search towards ~5-7V rated lamps.
