hives, usually from an allergic reaction to something eaten, but can also be from a bug bite/sting.
Has he had anything new to eat?
Best bet is to call the vet's office, explain the situation, ask if benadryl will help and how much. Also if he appears itchy, a cool bath helps.
Four days is a long time from a bite/sting. So I'd almost bet it's food or environment related and it's a series of reactions to the same trigger.
Let us know how it goes.