Option 1 states that an electrocardiogram (ECG) is a graphical representation of the electrical activity of the cornea. This statement is incorrect. The cornea is a part of the eye and has no electrical activity related to the heart. An ECG specifically measures the electrical activity of the heart and not any other organ.

Option 2 states that an ECG is a graphical representation of the activity of the kidney. This statement is also incorrect. The kidney is responsible for filtering blood and maintaining the balance of fluids and electrolytes in the body. It has no direct involvement in generating or regulating the electrical signals of the heart.

Option 3 states that an ECG is a graphical representation of the activity of the brain. This statement is incorrect. The brain is responsible for controlling various functions of the body through electrical signals, but it does not directly generate or regulate the electrical activity of the heart.

Option 4 correctly states that an ECG is a graphical representation of the electrical activity of the heart. It measures the electrical impulses generated by the heart`s electrical system, which are recorded as waves on the ECG graph. These waves help in diagnosing various heart conditions and abnormalities.

Hence, the correct answer is option 4.