I would like to power SIM800L with 4.0V as it is put in hardware documentation.
Power supply is stepdown module 12V to 5V.
I suppose just putting voltage divider wouldn't work good.
Has anyone done something similar?
You could use a zener diode or just a regular diode with VF of 1V. just remeber VFxI = diode power wasted. if its more than 1-2W you are going to have some problems.