Can a child get pvl at 4yo, is there any good way to prevent it?

No. Periventricular leukomalacia (pvl) is a form of white-matter brain injury, characterized by the necrosis of white matter near the lateral ventricles. It can affect newborns and (less commonly) fetuses; premature infants are at the greatest risk of the disorder. At 4 y/age child might have consequences related to pvl, such as cerebral palsy. There is no certain prevention known at this point.
Yes. This requires a long answer and there is an excellent description of the problem by sikply googling pvl. It is likely that a 4-year old had this problem at or near birth. Premature and low birth weight infants are more at risk.