- Ethnocentrism: The belief that one's own culture is superior, which can restrict cultural diversity by deeming other cultures inferior.
- Inbreeding: Reproduction within a limited gene pool, generally not contributing to cultural diversity.
- Cultural Relativism: Understanding and accepting different cultures, though not directly enhancing diversity.
- Immigration: The introduction of individuals from varied cultural backgrounds, thereby increasing cultural diversity.
Step 1: Understanding mechanisms of increased cultural diversity.
Immigration directly enhances cultural diversity by integrating individuals with distinct cultural practices, beliefs, and traditions.
Final Answer: \[\boxed{\text{The correct answer is (4) Immigration.}}\]