I am trying to power a RPi of 4 AA's, and my mobile power device can only output limited current (500mA), but I can underpower my RPi by frequency scaling. Is frequency scaling and current draw a linear relationship?
Generally no. There will be some component of current draw that is linear, which represents the switching losses of the CMOS logic. But there also be various static (constant) loads as well, including such things as CMOS leakage current, the operating current for other components operating at a constant frequency, pullup resistors, etc.