Ok, so War would have 2 arms, with a large number of spider like appendages on his back ending in an assortment of blades or claws. Tall, but not excessively muscular, using agility more than brute force.
Pestilence would be a corpse, covered with boils, vomiting blood, bloated, constantly wheezing, horribly disfigured.
Famine would be a thin corpse, like someone out of the pictures of Auschwitz survivors, with no teeth and spreading his weakness to all he touches
Death would be a shadow, never seen in detail, appearing and disappearing as he wishes, slaughtering all in his wake, amorphous and ambiguous.