Either define these variables globally outside of both functions (a very bad practice for any real world program - but suitable enough for an example) - or provide them as arguments to the alert function.

var one, two, three, four;

fun1();

function fun1 ()
{
  fun2();
  alert("var two + var three = " + two +  three);
}

function fun2 ()
{
  one = 1;
  two = 2;
  three = 3;
  four = 4;
}

fun2();

function fun1 (one, two, three, four)
{
  alert("var two + var three = " + two +  three);
}

function fun2 ()
{
  fun1(1, 2, 3, 4);
}

This is not possible to access a variable from outside of Scope

if you want to access it from more functions then make it global. Like below.

fun2();   
fun1();
var two = 2;
var three = 3;
function fun1() {
  alert("var two + var three = " + two + three);
}

function fun2() {
  var one = 1;
  two = two + 2;
  three = three +3;
  var four = 4;
}