That low voltage is put on the circuit by the Cruise Control box and/or the TECA. ..... not sure if its supposed to be 4v, but that is part of the circuit to make sure all the bulbs are working. Remember, the E4OD "watches" the brake lights, and if you have a bad bulb, it can mess with the transmission, like when the turn signal is on, the torque converter will lock and unlock in sequence the flashing light.....
Some people even see the LED light illuminate all the time, because the LED's take so little current to light, this small control current will make them light. A classic incandescent bulb take much more current to illuminate, but the filament provides a complete path for this control current, and everything is happy. This is where the resistors that Jasper mentioned come into play. They make the resistance of the circuit equal "electrically" to what the incandescent bulbs are with the LED's. This is also the same reason you see the turn signal flash really fast when LEDs are installed on an older vehicle. The turn circuit is not drawing as much current as it "should" so the flasher flashes faster, to indicate a burned out bulb...........
