What is the possibility of getting thyroid disease in a normal 4 year old child?

Low. Unless there is a family history of thyroid disease . Most states screen for thyroid disease at birth, ask your pediatrician to check your birth screening records at the state lab.
Thyroid. This is an unusual age but it could occur. Much higher chance in a female vs male. Higher likelihood if theres a strong family history in first-degree relatives ie mother, siblings, dad.
Not common. If there is a strong family history of thyroid disease, then it might be worthwhile checking with a blood test. In general it is uncommon for a normal four year old to just develop thyroid problems.