const unsigned int scan[] = {5, 6, 7, 8};
#define DATA 10
#define LATCH 11
#define CLK 12

byte numbers[] = {
  0xC0, 0xF9, 0xA4, 0xB0, 0x99,
  0x92, 0x82, 0xF8, 0x80, 0x90
};

void setup() {
  for (int i = 0; i < 4; i++) {
    pinMode(scan[i], OUTPUT);
    digitalWrite(scan[i], HIGH);
  }
  pinMode(DATA, OUTPUT);
  pinMode(LATCH, OUTPUT);
  pinMode(CLK, OUTPUT);
}

void show(int digits[10], int shift) {
  for (int i = 0; i < 4; i++) {
    digitalWrite(LATCH, LOW);
    shiftOut(DATA, CLK, MSBFIRST, numbers[digits[(shift + i) % 10]]);
    digitalWrite(LATCH, HIGH);
    digitalWrite(scan[3-i], LOW);
    delay(1);
    digitalWrite(scan[3-i], HIGH);
  }
}

void loop() {
  static unsigned long checkpoint = 0;
  static int digits[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
  static int count = 0;
  show(digits, count);
  if (millis() - checkpoint > 500) {
    count--;
    if (count < 0) count = 9;
    checkpoint = millis();
  }
}