Option 1 states that an endemic species is defined as a species whose population decreases in a forest due to a parasitic attack. However, this definition is incorrect. Endemic species are not necessarily affected by parasitic attacks or found only in forests.

Option 2 suggests that an endemic species is a cosmopolitan species commonly found in a biosphere. This definition is also incorrect. Endemic species are not cosmopolitan, meaning they are not found worldwide, and they are not commonly found in a biosphere as this would imply they are widespread.

Option 3 states that an endemic species is an endangered species found in a few restricted areas on Earth. This definition is close to the correct definition of an endemic species but is not entirely accurate. Endemic species are not necessarily endangered, but they are indeed restricted to specific areas on Earth.

Option 4 correctly defines an endemic species as a species confined to a particular region and not found anywhere else. This definition accurately describes the concept of an endemic species. These species have a limited distribution and are unique to a specific geographic region.

In conclusion, option 4 best defines an endemic species, while the other options provide incorrect or partially correct definitions.