Piriton contains chlorpheniramine which is an antihistaminic in nature. Chlorpheniramine and loratidine both are safe in a 4yr child. A definintive treatment for chicken pox contains oral acyclovir. It has to be started as early as the diagnosis is made to get a better outcome. Analgesic is also needed to get relief from pain. Fever is also have to control by using paracetamol. High fluid diet along with rest is also needed. You may use calamine lotion as before. Antihistamines like chlorpheniramine has mild role to get symptomatic relief. Chicken pox follows a natural time period of 7-12 days to complete its coarse. Weakness may present for a month too. If the symptoms are getting worse by time it recommended to visit a doctor for proper management.