I'll add to what archi already said. The current L4, is essentially an "E-Series" light, in that all it's parts are compatible (thread wise) with E-Series parts. What's currently termed as the "L-Series", mostly now refers to the L1, L2, and the Aviator, with essentially the two stage twisty mentioned already. The older discontinued "L" Series were simply LED itterations of existing lights.
L4 = LED E2e
L6 = LED M3
L7 = LED AN8 (Rechargeable)
If you really want a doozy, check out the A2Z...
Some of the L series heads will fit E-Series body, and vice versa, but not all, and the tail-caps of the L2/L1/A2 only work on those lights. Keep in mind, since with the L1/L2, the resistor is in the tail-cap, so putting an L2 head on an E2 body will only give you one level of light, since the L2 (and L1) tail-cap controls the two brightness setting, where as the E2L head, the two brightness levels are controlled from the head, and not the tail-cap.
One of my favorite combo's is the "E2B-AA" (E1B Head on an E2L-AA Body) or the E2L-AA-Rd (Red L1 Head on the E2L-AA Body).
Clear as mud? :0) Hope this helps!