Final answer:
The lead V4 for an EKG should be placed on the patient's left chest at the fifth intercostal space at the midclavicular line.
Step-by-step explanation:
The medical assistant should place the lead V4 on the left chest when preparing a patient for an EKG (electrocardiogram). Specifically, lead V4 is placed at the fifth intercostal space at the midclavicular line. It is one of the six chest leads used in a 12-lead ECG, which provides a comprehensive view of the heart's electrical activity by measuring voltages between pairs of these electrodes.