Slice potatoes in half lengthwise; place in large MICROWAVE SAFE baking dish. Pour 1/2 cup of the milk over potatoes. Microwave, covered, on HIGH 11 minutes. Remove baking dish from microwave. Move center potatoes to ends of baker and outer potatoes to center. Cover; microwave on HIGH 8-11 minutes or until potatoes are easily pierced with a fork. Remove from microwave. Coarsely mash potatoes, set aside.
Meanwhile, whisk cream cheese until smooth. Slowly add remaining 3 cups milk, whisking until smooth. Add cream cheese mixture and butter to baking dish. Microwave, covered, on HIGH 3-5 minutes or until mixture is hot.
Slice green onions. Carefully remove baking dish from microwave. Grate cheddar cheese over chowder using Rotary Grater; add green onions, salt and black pepper and mix until cheese is melted. Serve with toppings, if desired.