PREHEAT OVEN TO 250F. Wash but don't peel the sweet potato. Shred potato on fine blade of a shredder or food processor. Place in mixing bowl and set aside. Finely chop ham; pulse using puree blade of food processor. Add ham to the shredded potato. Add salt and pepper and mix well. Heat 1 tablespoon butter in a non-stick 6-inch skillet over medium heat. Add one-quarter of the potato-ham mixture. Cook about 4 minutes, pressing down gently to compact the hash as it cooks. Flip the hash patty and cook another 5 minutes. Add more butter if necessary. Repeat with remaining mixture; add more butter to skillet as needed. As each hash patty is done, transfer to oven to keep warm. Place 2 poached eggs on each serving. Garnish with a dollop of sour cream.