Your description of your four-year-old son is complex. I recommend that you meet with the preschool evaluation service of your neighborhood public school.
I cannot be specific about the cause. I can share your concern. You need to discuss these problems with the school professionals to explore what his problem areas are.
Your brother is very lucky to have a brother like you. Your awareness and sensitivity go beyond your 15 years of life. It sounds like both you and your parents need help.
It is important to differentiate the cause of the behaviors; however, the interventions are the same. I would suggest that the first step focus on why school is stressful.