Let's make sure we're talking about the same thing. I'm understanding you to say that you put the positive lead of your meter on the negative post of the battery and the negative lead of the meter on the frame and you're measuring 4 volts. While you did this, the battery cable was connected both to the negative post of the battery and to the frame.
Now, if what you're saying is that you disconnected the negative battery cable, then put the positive lead of your meter to the negative post of the battery and the negative lead of the meter to ground, then the 4 volts you're measuring makes no sense. You should be reading battery voltage in that case.
Regardless of any current drain on the system, the voltage from the negative terminal to ground, with the battery cable disconnected, should be almost exactly the same as if you measured from the positive post to ground. The internal resistance of most batteries can be measured in the tenths of an ohm (or effectively zero for most purposes.)
Since this happened recently, can you think of any changes you made to your electrical system within the same time frame?
