How to read particular line from *.txt file?

How can I get a string which is for example in 4th line of a text file?
Here’s the code I’m using to read/write/edit file:

import System.IO;
var filePath = "/Users/testWrite.txt";

function Update(){
    if (Input.GetKeyDown("c")) {
        WriteFile(filePath);
    }
	if (Input.GetKeyDown("w")) {
        AddToFile(filePath);
    }
    if (Input.GetKeyDown("r")) {
        ReadFile(filePath);
    }
}
function WriteFile(filepathIncludingFileName : String){
    var sw : StreamWriter = new StreamWriter("testFile.txt");
    sw.WriteLine("Line to write");
    sw.WriteLine("Another Line");
    sw.Flush();
    sw.Close();
}
function AddToFile(filepathIncludingFileName : String){
    sr = File.AppendText("testFile.txt");
    sr.WriteLine("Added ");
    sr.Write("Another Line");
    sr.Flush();
    sr.Close();
}
function ReadFile(filepathIncludingFileName : String) {
    sr = new File.OpenText("testFile.txt");
    input = "";
    while (true) {
        input = sr.ReadLine();
        if (input == null) { break; }
        Debug.Log("line="+input);
    }
    sr.Close();
}

You could read the whole file into a list and then you could access any line you like:
import System.Collections.Generic;
import System.Linq;

  var fileLines : List.<String>;

  function ReadFile() {
     var sr = File.OpenText("whatever.txt");
     fileLines = sr.ReadToEnd().Split("

"[0]).ToList();
sr.Close();
}

Now you have a list of lines that you can insert into, get a specific line from - whatever. If you want to write them back out you can just do:

 var wholeFileText = String.Join("

", fileLines.ToArray());