What causes 4th nerve rt upper quadrant palsy in children?

Congenital. Usually congenital-idiopathic. Rarely a brain tumor. Sometimes post-viral infection. But surely you discussed this with the ophthalmologist, no?
Congenital or due to. A problem with the 4th cranial nerve (which controls one of the muscles that move the eyeball) can be congenital, meaning the child is born with it (caused by incomplete or abnormal development of the nerve). A child who later acquires a 4th nerve palsy can get the palsy from head trauma, increased pressure in the brain, brain infections, nerve demyelination, tumors, etc....