1. Oxygen (O): Found in water, carbohydrates, lipids, proteins, and nucleic acids. It's essential for respiration and energy production.

2. Carbon (C): The backbone of all organic molecules, including carbohydrates, lipids, proteins, and nucleic acids. It forms the framework for life.

3. Hydrogen (H): Found in water, carbohydrates, lipids, proteins, and nucleic acids. It plays a crucial role in chemical reactions and energy transfer.

4. Nitrogen (N): A key component of proteins and nucleic acids (DNA and RNA). It's also found in some lipids.

These four elements make up over 99% of the mass of most organisms.