Just speaking out loud as I'm further learning / understanding this stuff:
In a regulated light like the TK70, it needs 9.6 amps I think. So with 4 D cells that is 2.4 amps per D cell. I think it needs like 4.8v so each cell is providing its correct 1.2v. But with under that heavy load, if a cell is not capable of producing enought volts per amperage drain, it will cause the light to kick down from turbo to high (the light's driver does the kick down as soon as not enough volts from the battery pack is not avaiable anymore.