For single phase, 220v ac: what is the difference between 3 and 4 wire
systems?

Most 220 comes into houses with three wires. Two insulated, and one bare.
This implies to me that the bare one is also a ground, as well as the 220v
neutral.

I also observe portable generator to house setups using 4 conductor cable.
Why the difference? Are the 4 conductor setups simply using separate
neutral and ground? If so, is there any practical and/or safety reason to
separate the neutral from ground lead?