There's another method for 'governing' engines as well, but this falls more under the heading of 'detuning' - I'll include it for completeness.
5) Over-rich (conservative) carb jetting. Engines are set up a little too rich, right from the factory. This works to limit output power, but also provides a safety margin for the manufacturer. The richness causes the engine to run cooler, which makes the possibility of a piston/cylinder failure much less likely. You get less power, and the manufacturer gets less warranty returns.