Answer 1

A simple method would be to convert it back to int, add your number, then convert back to string

Answer 2

You can also get help from std::stoi()

std::string n { std::to_string( 4 ) }; // "4"

n = std::to_string( std::stoi( n ) + 4 ); // "8"

Answer 3

If you know that there will be no carry when you add, then you can shift the character value by 4.

string n = "4";
n[0] += 4;
cout << n; //output: "8"