analog to digital demux?
4 bit Analog to Digital


What you are looking for is typically called a Flash A/D.
You use comparators to detect the input signal level and convert this to your required 4 bit digital output. 

Shown below is a 3 bit convertor, but you'll get the idea:



There are many flash convertors available and you can get ideas for implementations from devices such as the ADC0820, a decades old workhorse.