First of all, jssp78, welcome to CPF!
Your question was actually a very popular one, a little while ago.
not possible.
(short answer to a short question)
Actually, the answer is a little more complicated than you think. The answer is actually a yes. You can achieve 2 stage operation on an L4. Read on.
You can put a McE2S into a tailcap (the McE2S is available from the Sandwich Shoppe website, they have a forum here on CPF) which will make the L4 into a 2 stage light. The tailcap will operate in the same fashion as the L1, L2 and the A2. That is, twist slightly for low, more for high. You can have constant on and momentary etc as per the L4/A2/L1.
Unfortunately the price you pay for this is decreased runtime on the lower level, since the McE2S uses a resistor to drop the L4 out of regulation and is therefore less efficient. FYI, the L2 uses 2 driver boards, one for each of its levels. The driver boards are aligned parallel to the long axis of the light.
The McE2S comes in a variety of resistances, which allow you to choose the brightness of your lower level.