You have a problem, however. The 4.0v value you want to subtract may or may not be constant. Does it change with temperature? Will it drift over time (hours, or even years)? Odds are that it will change. What you want is a way to subtract any mostly-DC signal, not just subtract 4.0000v.
Fortunately, there is a simple solution. Use a capacitor! When used in this application, it is called a DC Blocking Cap and it is very common to see them in audio circuits. Basically, a cap has an almost infinite resistance at 0 Hz, and the resistance decreases as the frequency gets higher. Here is an example circuit:
simulate this circuit – Schematic created using CircuitLab
The resistor is required to "restore" the DC value of the audio signal. If we didn't have a resistor then the DC level could end up to be something we don't want. In this case we're grounding the other side of the resistor so that our DC value will be 0v. Sometimes you might want the DC level to be something other than 0v, and you can do that by connecting the resistor to something other than ground.
Sometimes, when used in an opamp circuit, the resistor is optional because something with the rest of the opamp circuit fulfills that role.
This circuit is also a passive high-pass filter, and the cutoff frequency depends on the values of the C and R. The higher the values, the lower the cutoff frequency will be. I would start with 22 uF and 20K, but you could easily get away with 4.7u and 10K. For audio applications we try to get the cutoff frequency of this filter to be below 20 Hz.
The normal type of cap to use is an Aluminum Electrolytic. These caps are polarized, but are often used in an unpolarized way for this application. While technically not "correct", it is an accepted practice in consumer and most pro-audio products so long as you are not running significant power through the cap. Even so, try to orient the cap so that it is "more correct".
For your application, you can place a DC blocking cap somewhere in your signal chain. Maybe even in several places. I can't say exactly where, since you didn't give us schematics of your circuit. But my first guess would be after the transistor based preamp.