设计一个子函数，将任意n×n的矩阵转置，并在主函数中调用此函数将一个4×4的矩阵转置，并在主函数中输出结果。

以下是一个实现矩阵转置的示例代码：

def transpose_matrix(matrix):
    n = len(matrix)
    for i in range(n):
        for j in range(i, n):
            matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

# 测试代码
matrix = [[1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12],
          [13, 14, 15, 16]]

transpose_matrix(matrix)

for row in matrix:
    print(row)

输出：

[1, 5, 9, 13]
[2, 6, 10, 14]
[3, 7, 11, 15]
[4, 8, 12, 16]

在主函数中调用此函数并输出结果：

matrix = [[1, 2, 3, 4],
          [5, 6, 7, 8],
          [9, 10, 11, 12],
          [13, 14, 15, 16]]

transpose_matrix(matrix)

for row in matrix:
    print(row)

输出：

[1, 5, 9, 13]
[2, 6, 10, 14]
[3, 7, 11, 15]
[4, 8, 12, 16]