Melt 2 tablespoons butter, saute celery and onions. Add potatoes and enough water to cover Simmer until potatoes are tender. Melt remaining 2 tablespoons butter; blend in 2 tablespoons flour to make a roux. Add roux and evaporated milk to potatoes. Heat until thickened over medium heat stirring constantly. Add seasonings, salmon and tomatoes. Heat until steamy. Do not boil. Add cheese just before serving.