4*4 Matrix Keyboard program

Hello everybody !

I have a little problem :

I actually have a matrix keyboard like this one :

And I am using the following program :

#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {5, 4, 3, 2}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {9, 8, 7, 6}; //connect to the column pinouts of the keypad

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 

void setup(){
  Serial.begin(9600);
}
  
void loop(){
  char customKey = customKeypad.getKey();
  
  if (customKey){
      Serial.println(customKey);
  }
}

I am using the keypad Library.

Here is my actual problem : for the moment, the program can only get one number (or letter or * or #).
And I would like a program where we can save several numbers in différents ‘‘int’’ so we can create a whole security code (like 4521 or 7846).

Can someone help me ? I am still pretty new with keypads and I feel a bit lost with it.

Create a char buffer and each time a new key is pressed add it at the end of the buffer until the buffer is full?

How about using Google with "arduino keypad password tutorial"? The 39500 results might contain something useful.