Final answer:
The central sulcus serves as a major landmark dividing the frontal lobe from the parietal lobe in the cerebral cortex. Hence option 4 is correct.
Step-by-step explanation:
The external space that provides a major landmark dividing one cerebral lobe from another within the same hemisphere is the central sulcus. This landmark separates the frontal lobe from the parietal lobe, two of the four major regions of the cortex. Other sulci, such as the lateral sulcus and the parieto-occipital sulcus, serve as landmarks for other lobe boundaries. The central sulcus is a critical landmark for understanding brain anatomy and function, as it distinguishes between areas involved in sensory processing and motor control.