Option 1: Bow legs, knock knees, and pigeon chest - These symptoms are typically associated with a condition called rickets, which is caused by Vitamin D deficiency rather than protein deficiency. Rickets is characterized by weak bones and skeletal deformities.

Option 2: Coughs, colds, and fever - These symptoms are commonly associated with respiratory or viral infections and are not directly related to protein deficiency.

Option 3: Fever and rashes - While fever and rashes can occur in various infections, they are not specifically linked to protein deficiency.

Option 4: Stunting of growth, discoloration of hair, and mental apathy - These are classic symptoms of protein deficiency during the pre-school age. Insufficient protein intake can hinder growth and development, leading to stunted growth. Hair discoloration can occur as a result of inadequate protein synthesis. Mental apathy refers to a lack of energy, attention, or motivation, which can be caused by a lack of essential amino acids that are necessary for optimal brain function.

Therefore, the correct answer is option 4.