Take a look at: http://www.gammon.com.au/forum/?id=11916
That shows using the analog comparator.
You set up a reference voltage (eg. 4V in your case) on one pin and the comparator gives an interrupt on the rising, falling or both edges (on another pin).
I would like to be able to read the exact value of analog pin ...
It will be at or about the reference voltage. What else would it be?
I want a program to execute a certain code, whenever the voltage on analog pin drops anywhere below 4V
You can execute code but interrupt routines are supposed to be short.