Will the non tw4 KL4 head light up on a single primary?
There really is no such thing as a "non-tw4 KL4". You have the original KL4, which used the Luxeon Lux-V led. That was what the TW4 was built with, and it will work with both CR123's and R123's. Then you have the newer KL4, which is the KL4A, using the SSC LED, which will not run on a primary since it's regulated (the Lux-V was direct drive). The SSC KL4 will work with a R123 though. So, if you want to build one, you need to buy a Lux-V KL4, which, IMHO, is the better of the two for building a TW4. Hope this helps.