Thanks for your query,
in children is usually due to dietary factor.
Since he is 4 years of age and is asymptomatic hypothyroidism
is ruled out.
There is also no problem of fissures also.
I suggest you to modify his diet;
Give plenty of fluids,salads,green leafy vegetables.fruit .
Avoid all kind of fried and junk food.
Wishing your child good health.