There isn't a fixed set of "four units of speed". Speed is a measure of distance traveled over a period of time, and there are many units used to express it.

Here are some common units of speed:

* Metric:

* Meters per second (m/s): The standard unit in physics.

* Kilometers per hour (km/h): Used for cars, trains, etc.

* Imperial:

* Miles per hour (mph): Used in the US and some other countries.

* Feet per second (ft/s): Used in some engineering applications.

* Nautical:

* Knots (kn): Used for nautical speeds, one knot is equivalent to one nautical mile per hour.

These are just a few examples, and there are many other units used depending on the context.