Based on the weight of your dog (about 15-17kg) I would say no more than one 4mg tablet per day, but don't take my word for it!
When using Piriton you are always advised to see a vet first because the drug can have many side effects including vomiting, diarrhoea, and drowsiness. Piriton may also interact with other medications that Katie may be taking, and in rare cases the dog may be allergic to the drug.
Try to keep Katie dry over the weekend in the meantime - if the skin gets wet it may make it more itchy!