I have both an L2 and an L4. I replaced the L4's tailcap with a McGizmo 2-stage and what I think is a 22-ohm resistor. I use AW's 1600 mAh 17670 Li-Ion cells.
The difference between my larticular flashlights is that the L4 is shorter, can tailstand, has no regulated low, and uses both primary cells and rechargeables. Brightness on both high and low is essentially the same, as is runtime (I expect, testing that now).
For my use, I don't need a regulated low. The bright level is the important one to have regulated. You want max capability every time you punch the button, and don't want to be at the mercy of your batteries. Having the low setting gradually dim is of almost no consequence.
One thing I didn't think about was that the McGizmo switch will change brightness in low when you go from primaries at 6 V to Li-Ion at 3.7 V. (Doh!) The 60-ohm resistor was about right with primaries, but I swapped it for the only other flavor I had -- 22-ohm -- when I discovered that my low mode had gone from bright enough to walk by to dimmer than a SureFire L1's low.
Scott
PS: Yes, Kiessling, I guess you are right. My L4's low is not quite as bright as the L2's. I would not rate it any less functional though.