The TK40 has made me want a small version!

I love the Fenix TK40! I think it the most near perfect light for me that I can think of. It is amazingly bright, has insane runtimes, is the perfect size for most purposes, and uses safe, readily available batteries. This has given me an idea! The TK40 uses 8 AA batteries...two sets of 4. Each set of 4 batteries (say Eneloops or their equivalents)are connected in series, and throw 4.8 volts to the LED. The two sets are connected in parallel, which doubles the capacity rating of the battery "pack" it creates. In essence, we have a 4.8v , 4200 Mah battery pack which powers this light saber. Somebody that may more about batteries and electricity please feel free to correct me . How about a smaller version? Say 4 AA's, two sets of 2 cells connected in series, then those two sets connected in parallel. This would give us a high end AA light, cranking out about 180-220 lumens on high, but instead of an hour an a half run time, we would get about 3 hours. The form factor of the light seems to me would be very comfortable in the hand! Does a light like this exist???

It doesn't exist by Fenix yet, but other makers make AA in parallel. One I can think of on top of my head is the Streamlight Propolymer 4AA lux.
It has a polymer tube, which is easier to make compared to boring out aluminium. The SL4AA propoly is very comfortable to hold in the hand, and ashame there isn't more around like it.

Could you keep the TK40 on high and get 6+ hrs out of it. The extra length of AA doesn't bother me at all. The thing that TK40 bothers me with is its lack of material around the bezel as well as a longer strike edge to both both the light and to acts as defensive tool.

It doesn't matter how the batteries are arranged, the total energy stored by the batteries is the same. Energy is the key quantity that relates to runtime, not capacity. e.g. In the TK40, assuming the LED driver electronics were equally efficient, 9.6V 2100mAh would have the same runtime as 4.8V 4200mAh.

BTW the SLPP4AA uses 4 AA cells in series.