To solve this problem, we need to identify the main components of Nissl bodies.$\newline$Nissl bodies are found in the cell bodies of neurons and are important for protein synthesis.$\newline \bullet$Nissl bodies are composed of rough endoplasmic reticulum (RER) and free ribosomes.$\newline \bullet$The RER is involved in the synthesis of proteins that are either secreted from the cell or incorporated into the cell's plasma membrane.$\newline \bullet$Free ribosomes are responsible for synthesizing proteins that function within the cytosol.$\newline$Therefore, the correct answer is that Nissl bodies are mainly composed of free ribosomes and RER.$\newline$This corresponds to Option 4.