I was out in the yard tonight playing with my Maglite 4D with the new Terralux TLE-300M-EX drop in upgrade and I *finally* figured out the UI for changing the light output modes. To go from high-medium-low, you have to press the on/off switch very quickly. If you don't have a fast thumb, it may be hard to change the modes at first. It just takes some getting used to. It's not a huge deal though because it will default to the high mode after the light is left off for a short while as I mentioned before.
Note that the TLE-300M-EX has a rather "floody" beam pattern. It's not a tight, pencil beam at all because of the three LEDs. I like floody beams though, so it's just what the doctor ordered for my Maglite because it has a wide beam without the much despised donut hole that all stock Maglites produce when set on flood.
For comparison, the TLE-300M-EX beam's hot spot is much wider than that for my Fenix TK40.