Write a C++ code to find largest among 4 numbers  using the concept call by reference.

Sure, here is a simple C++ code that uses the concept of call by reference to find the largest among 4 numbers:

#include<iostream>
using namespace std;

void findLargest(int &a, int &b, int &c, int &d, int &largest) {
    largest = a;
    if(b > largest)
        largest = b;
    if(c > largest)
        largest = c;
    if(d > largest)
        largest = d;
}

int main() {
    int num1, num2, num3, num4, largest;
    cout << "Enter four numbers: ";
    cin >> num1 >> num2 >> num3 >> num4;
    findLargest(num1, num2, num3, num4, largest);
    cout << "Largest number is: " << largest << endl;
    return 0;
}

In this code, we have a function findLargest that takes four numbers and a reference to a variable largest as arguments. It compares each number with largest and if the number is greater than largest, it updates largest. After the function call in main, the largest number is stored in the largest variable and is printed out.