The correct answer is option 4, South Korea.

Option 1, Cambodia, is a member of ASEAN.

Option 2, Malaysia, is also a member of ASEAN.

Option 3, Singapore, is a founding member of ASEAN.

However, option 4, South Korea, is not a member of ASEAN. ASEAN, which stands for the Association of Southeast Asian Nations, is a regional organization comprising 10 Southeast Asian countries. The goal of ASEAN is to promote regional cooperation and economic integration among its member states. Therefore, it is important to differentiate between ASEAN member countries, such as Cambodia, Malaysia, and Singapore, and non-member countries like South Korea.