Array using keypad

hy, I am having problems with my code. i have declared four numbers, and i want to enter each number on a keypad and compare it to the declared array. if its there then print that number on lcd. here is my code

#include <Wire.h> 
#include "DFRobot_RGBLCD.h"
#include <Keypad.h>

#define MachineNumber 5
char Machine[MachineNumber] = {'1','2','3','4'}; 
char customKey;


const byte ROWS = 4;
const byte COLS = 4;

char hexaKeys[ROWS][COLS] = {
  {'1', '4', '7', '*'},
  {'2', '5', '8', '0'},
  {'3', '6', '9', '#'},
  {'A', 'B', 'C', 'D'}
};

byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

DFRobot_RGBLCD lcd(16, 2);  

void setup(){
  lcd.init(); 
  
 
}

void loop(){
  lcd.setCursor(0,0);
  lcd.print("ENTER MACH NO.");

  customKey = customKeypad.getKey();
  if(customKey = '1' || '2' || '3' || '4' ){
    lcd.setCursor(0,1);
    lcd.print(customKey);
    delay (1000);
    lcd.clear();
    
    
    }
  
  
  }
#include <Wire.h> 
#include "DFRobot_RGBLCD.h"
#include <Keypad.h>

#define MachineNumber 5
char Machine[MachineNumber] = {'1','2','3','4','5'}; 
char customKey;


const byte ROWS = 4;
const byte COLS = 4;

char hexaKeys[ROWS][COLS] = {
  {'1', '4', '7', '*'},
  {'2', '5', '8', '0'},
  {'3', '6', '9', '#'},
  {'A', 'B', 'C', 'D'}
};

byte rowPins[ROWS] = {9, 8, 7, 6};
byte colPins[COLS] = {5, 4, 3, 2};

Keypad customKeypad = Keypad(makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS);

DFRobot_RGBLCD lcd(16, 2);  

void setup(){
  lcd.init(); 
  
 
}

void loop(){
  lcd.setCursor(0,0);
  lcd.print("ENTER MACH NO.");

  customKey = customKeypad.getKey();
  for(byte i = 0; i < MachineNumber; i++) {
    if(customKey == Machine[i]){
      lcd.setCursor(0,1);
      lcd.print(customKey);
      delay (1000);
      lcd.clear();
    }    
  }  
}

By the way, in your original code, you make the array to contain 5 numbers but you only gave 4.

is it possible to extend it to input two digit numbers.

Yes, its possible.

If the user presses "1" and then "2", how will the Arduino know if they are asking about machine 1 and machine 2, or machine 12?

Maybe one of these links will help