In 4 wire sensors there are two ground wires to get more accurate readings than the old single wire standard, the other two wires are signal and heater power. This arrangement also helps make more accurate readings because the old single wire sensors could only get a decent reading above a certain temperature, which meant it had to happen above idle. The heated O2 sensors see much less fluctuation, and thus are more stable for the ECU to use for tuning. This is especially true on engines like the WRX, which has a fuzzy logic system to optimize the fuel and spark patterns based on how the car is driven.

A true "wide band" O2 sensor isn't much different from the 4 wire sensors, but have a 5th wire that I forget the purpose of at the moment. Wide band O2 sensors are more adept at measuring the presence of other gases in the exhaust though than production O2's.