const byte numbers[10] = {
  0b00111111,
  0b00000110,
  0b01011011,
  0b01001111,
  0b01100110,
  0b01101101,
  0b01111101,
  0b00000111,
  0b01111111,
  0b01101111
};

int digits[4] = {0,0,0,0};
unsigned long lastUpdate = 0;
const unsigned long interval = 700;

void setup() {
  DDRD |= 0b01111111;
  DDRB |= 0b00001111;
  PORTD &= ~0b01111111;
  PORTB |= 0b00001111;
}

void loop() {
  if (!(digits[0]==0 && digits[1]==0 && digits[2]==3 && digits[3]==3)) {
    if (millis() - lastUpdate >= interval) {
      lastUpdate = millis();
      digits[3]++;
      if (digits[3] > 3) {
        digits[3] = 0;
        digits[2]++;
        if (digits[2] > 3) digits[2] = 0;
      }
    }
  }

  for (int i = 0; i < 4; i++) {
    PORTB |= 0b00001111;
    PORTD = (PORTD & 0b10000000) | (numbers[digits[i]] & 0b01111111);
    PORTB &= ~(1 << i);
    unsigned long start = millis();
    while(millis() - start < 5);
  }
}