Reading 4.2v with the ADC on an ESP8266-12E
The raw ESP ADC has a full scale range of 0-1V, so you would need a divider of about 4.2:1 to capture the 4.2V input. Since you're switching it, the resistance value is not critical and can be made reasonably low. 

Perhaps something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

The ADC sees a source impedance of about R2||R3 = 11.5K when measuring (15K when off, but that doesn't matter) and the 0-1V
ADC input range translates into 0-4.32V. You might want to make the divider ratio
a bit more conservative since the ADC in that part is so very loosely specified. 

This circuit draws less than 110uA when measuring and almost nothing when 'off'.