volatile int counter = 0;

void setup() {
  Serial.begin(9600);
  pinMode(4, INPUT_PULLUP);
  pinMode(5, INPUT_PULLUP);
  PCICR |= B00000100; // Enable interrupts on PD port
  PCMSK2 |= B00110000; // Trigger interrupts on pins D4 and D5
}
void loop() {
  Serial.print("The counter is now at: ");
  Serial.println(counter);
  delay(1000);
}
ISR (PCINT2_vect) {
  if (PIND & B00010000) {
    counter++;
  }
  else if (PIND & B00100000) {
    counter--;
  }
}