C++ I need just a function that will take a constant reference to a string and will return a string

C++ I need a function that will take a constant reference to a string as its parameter and will return a string. The function should look through the string and return any repeated sequence of 4 digits. If there isn't a repeated sequence of 4 digits exact, return a empty string.

For example: If the inputted string is "397390362927390", the function should return "7390" as it is repeated twice. At most there will be one repeated sequence in the input.

This is what I have so far.

   string find_repeated(const string &line){
     string a = line;
     if (a.length() <= 4){
       for (string::size_type i=0; i < a.length(); i++) {
         return a;
       }
     }  
   }