在每个字符串的第4个索引中添加一个点

def add_dot(strings):
    modified_strings = []
    for string in strings:
        if len(string) >= 4:
            modified_string = string[:4] + '.' + string[4:]
            modified_strings.append(modified_string)
        else:
            modified_strings.append(string)
    return modified_strings

# 示例输入
strings = ['example', 'test', 'hello', 'world']

# 调用函数并打印结果
modified_strings = add_dot(strings)
for string in modified_strings:
    print(string)

exam.ple
test
hell.o
worl.d