The 4th wire is not for powered microphones. Powered microphones use what is called "Phantom power". This is DC voltage that is applied to the signal wires (pins 2 and 3).
The 4th connection on the XLR plug is connected to the equipment chassis/ground. This can actually be different than Pin 1 signal ground (which is a common/shared ground for the analog circuits), but not necessarily a different connection on all equipment.
The 4th connection on the XLR plug is connected to the equipment chassis/ground. This can actually be different than Pin 1 signal ground (which is a common/shared ground for the analog circuits), but not necessarily a different connection on all equipment.