A shamrock has three leaves by definition. Anything with four-leaves, clover, medic, wood sorrel, or anythingthing else is not a shamrock.
A three-leaf (normal) clover may be considered a shamrock. Sometimes medic or wood sorrel is also deemed a shamrock. There are also some other plants that are named shamrock plants.
The shamrock has been registered as a trademark by the Government of Ireland.