Final answer:
The 4th NREM sleep stage is known as deep sleep or stage 3 sleep, characterized by delta waves and significant reductions in heart rate and respiration (Option 4),
Step-by-step explanation:
The term used to refer to the 4th NREM sleep stage is deep sleep. During this stage (stage 3 of NREM), which can be called stage 3 sleep, there is a predominance of low frequency, high amplitude delta waves. This stage is characterized by a state where the heart rate and respiration slow dramatically, and the sleeper is less responsive to external stimuli, making it more difficult to awaken them.
Stage 3 is often mixed up with the concept of a fourth stage, but in the current sleep stage classification, NREM sleep is now usually divided into three stages, with the third stage encompassing what was previously described as stages 3 and 4. REM sleep is a different stage marked by rapid eye movements and brain waves similar to wakefulness.
Thus, the correct option is number 4.