int pins[] = {18, 19, 21, 22};

void setup() {
  for(int i = 0; i < 4; i++) {
    pinMode(pins[i], OUTPUT);
  }
}

void stepForward() {
  digitalWrite(pins[0], HIGH);
  delay(5);
  digitalWrite(pins[0], LOW);

  digitalWrite(pins[1], HIGH);
  delay(5);
  digitalWrite(pins[1], LOW);

  digitalWrite(pins[2], HIGH);
  delay(5);
  digitalWrite(pins[2], LOW);

  digitalWrite(pins[3], HIGH);
  delay(5);
  digitalWrite(pins[3], LOW);
}

void loop() {
  for(int i = 0; i < 200; i++) {
    stepForward();
  }
  delay(1000);
}