My son is 4years old in his body there are so much hairs ... what should i have to apply

Hi as he grows it will fall off , don't worry. Just make sure to keep his skin soft and moisturized. Use good medicated soap like Bioderma Atoderm.

this is very normal. my baby also had very good growth of ruwaa on his body then I use to maalis with mamaearth soothing Masaage oil and then used give bath and used to rub with muslin cotton. all the ruwaa gone away in 2 months.