What you have is a Dead-Short in the 4th fitting.
Mike,
How do you figure that? If there was a dead short the other lights would go out.
I go with Dereck's answer: There is a bad (high-resistance) connection somewhere. When you just put the meter at the 4th lamp terminals it's drawing very little current, so you see the 12 volts. As soon as you connect the low-resistance of the lamp, the high-resistance of the bad connection becomes the dominant part of the circuit, so the voltage at the lamp drops to zero (or very nearly so).
