Running 3,6V MCU on full 4,25V Li-ion battery
A silicon rectifier in series with the battery will drop about 0.7V, but will need a minimum load to be sure that it has that drop. Be certain to get a rectifier rated for the expected current draw.

For a 1N400x (0.7V drop at 10mA):

\${{4.2\text{V}−0.7\text{V}} \over 10\text{mA}} = 350 Ω\$

Therefore you would require a 350Ω resistor in parallel with the load to maintain the voltage drop.