Arduino door lock

#include <Keypad.h>
#include <Wire.h>
#include <LCD.h>
#include <LiquidCrystal_I2C.h>

#define I2C_ADDR    0x27  // Define I2C Address where the PCF8574A is
#define BACKLIGHT_PIN     3
#define En_pin  2
#define Rw_pin  1
#define Rs_pin  0
#define D4_pin  4
#define D5_pin  5
#define D6_pin  6
#define D7_pin  7


LiquidCrystal_I2C  lcd(I2C_ADDR, En_pin, Rw_pin, Rs_pin, D4_pin, D5_pin, D6_pin, D7_pin);

const int beeper = 11;

const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns
char keys[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

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

const String valid = "7A8C";
String entry = "";

void setup() {
  Serial.begin(9600);
  lcd.begin (20, 4);

  // Switch on the backlight
  lcd.setBacklightPin(BACKLIGHT_PIN, POSITIVE);
  lcd.setBacklight(HIGH);
  lcd.home();
  lcd.print(" SECU-SURE SECURITY");
  delay(2000);
  tone(beeper, 987);
  delay(100);
  tone(beeper, 1318);
  delay(300);
  noTone(beeper);
  lcd.setCursor ( 0, 1 );
  lcd.print("  >>>> SECURE <<<<  ");


}

void loop() {
  char key = keypad.getKey();
  if (key) {
    tone(beeper, 3000);
    delay(50);
    noTone(beeper);
    Serial.print(key);
    if (entry.length() == 3) {
      delay(200);
      lcd.setCursor(7, 3);
      lcd.print("*");
      entry = entry + String(key);
      Serial.println("");
      lcd.setCursor ( 0, 3 );
      lcd.print("              ");
      if (entry == valid) {
        lcd.setCursor ( 0, 1 );
        lcd.print("  >>>> UNLOCK <<<<  ");
        //do things
        tone(beeper, 3000);
        delay(50);
        noTone(beeper);
        delay(50);
        tone(beeper, 3000);
        delay(50);
        noTone(beeper);

        delay(5000);
        lcd.setCursor ( 0, 1 );
        lcd.print("  >>>> SECURE <<<<  ");
      } else {
        tone(beeper, 400);
        delay(1500);
        noTone(beeper);
      }
      entry = "";
    }
    else if (entry.length() == 2) {
      lcd.setCursor(6, 3);
      lcd.print("*");
      entry = entry + String(key);
    }
    else if (entry.length() == 1) {
      lcd.setCursor(5, 3);
      lcd.print("*");
      entry = entry + String(key);
    }
    else if (entry.length() == 0) {
      lcd.setCursor(0, 3);
      lcd.print("PIN:");
      lcd.setCursor(4, 3);
      lcd.print("*");
      entry = String(key);
    }
  }
}