怎么用python批量在txt文件内容中第四行加前缀呀

求解答，怎么用python批量给txt文件中第四行添加前缀呢？需要批量添加两个数字。该怎么写呢。

import glob

def main():
    prefix = "XXX"
    for fname in glob.glob("*.txt"):
        with open(fname, "r") as f:
            lines = f.readlines()
        if len(lines) >= 4:
            lines[3] = prefix + lines[3]
            with open(fname, 'w') as f:
                f.writelines(lines)

if __name__ == '__main__':
    main()