4 yr old had sleep study that showed decreased rem sleep &frequent wakings &arousals (7.5 per/hr).she also snores, sleep talks &walks, wets bed.causes?

Sleep disorder: Although there is an entity called central sleep apnea (central nervous system, brain) the most common cause of a sleep disorder in a 4 year old is hypertrophy (enlargement) of the tonsils and adenoids. Removal of these structures in the upper airway almost always results in a cure of these symptom.

See an ENT: The most likely problem is that you 4 year old is experiencing obstructive sleep apnea(usually to large tonsils and adenoids). All the symptoms and signs you are describing (including wetting the bed) can be improved with treatment(removal of tonsils and adenoids). They often get better the first night after surgery.