{4, 5, 6, 7} has 4 elements, so there should be 2⁴ = 16 subsets.
• empty set: { }
• each set containing only 1 element: {4}, {5}, {6}, {7}
• each set containing 2 elements: {4, 5}, {4, 6}, {4, 7}, {5, 6}, {5, 7}, {6, 7}
• each set containing 3 elements: {4, 5, 6}, {4, 5, 7}, {4, 6, 7}, {5, 6, 7}
• the set itself: {4, 5, 6, 7}
Count them up to make sure we have them all:
1 + 4 + 6 + 4 + 1 = 16
