Single hair is normal However needs evaluation Kindly consult pediatrician

It not normal but if the baby is having no other symptoms or sign , then don’t worry.

A single silver (white/gray) hair in a 4-month-old baby is usually not a cause for concern. It can be due to: Possible Causes like Genetic variation,Nutritional deficiency (rare): Deficiency of vitamin B12, iron, or copper could lead to hair depigmentation,Waardenburg syndrome (very rare)-  If associated with hearing loss, wide-set eyes, or white patches on the skin and Piebaldism (rare)-  congenital condition causing white patches in hair or skin.

Usually it is normal Don't pluck or cut Next hair would be normal

Consult with Pediatrician physically for further evaluation and treatment