Your approach of looking for the root cause of the behavior is quite thoughtful. The child reminds me a bit of myself around that age. I was nonverbal until I was 4 years old and for a short time I was in a preschool with other kids and they would have nap times, cribs for the tiny kids and various types of toys. I couldn't stay there because I would get agitated and try to bite. In my own case that was because I was getting regularly abused at home, including being gagged to keep me quiet. So if I would see an adult standing over me, including when I'd wake up, I would flat-out panic. I was afraid if it seemed someone might grab or hit me. Sensory overload always has made me cranky (even to this day), but never aggressive. I would imagine a child could have a meltdown due to sensory issues, but your description of his behavior sounds more directed than that. Does the 4 year-old have a place where he can nap just by himself, where there won't be adults around? His behavior upon waking alone might help to determine if he's reacting to sensory overload (the noise and lights) versus reacting to the presence of adults due to confusion and arousal of his fight-or-flight response.