Final answer:
For a 4-month-old infant with vomiting and dehydration, the best position recommended by a nurse is the side-lying position to reduce the risk of aspiration.
Step-by-step explanation:
In determining the appropriate position for a 4-month-old infant diagnosed with vomiting and dehydration, the nurse should consider the developmental capabilities of the infant as well as safety measures to reduce the risk of aspiration. At this age, an infant can lift their head while lying prone and start to roll from front to side. However, when caring for an infant with vomiting, the side-lying position is typically recommended to decrease the risk of aspiration. Therefore, the nurse would suggest positioning the infant in a side-lying position, as it allows fluids or vomitus to drain away from the airway, reducing the risk of inhalation.