int A = 11; // was 8
int C = 10; // was 9
int B = 9;  // was 10
int D = 8;  // was 11

void setup() {
  Serial.begin(9600);
  pinMode(A, OUTPUT);
  pinMode(B, OUTPUT);
  pinMode(C, OUTPUT);
  pinMode(D, OUTPUT);
}

void loop() {
  
  digitalWrite(D, HIGH);
  digitalWrite(A, LOW);
  digitalWrite(B, LOW);
  digitalWrite(C, LOW);
  delay(20);

  digitalWrite(D, LOW);
  digitalWrite(C, HIGH);
  digitalWrite(B, LOW);
  digitalWrite(A, LOW);
  delay(20);

  digitalWrite(D, LOW);
  digitalWrite(C, LOW);
  digitalWrite(B, HIGH);  
  digitalWrite(A, LOW);
  delay(20);

  digitalWrite(D, LOW);
  digitalWrite(C, LOW);
  digitalWrite(B, LOW);
  digitalWrite(A, HIGH);
  delay(20);
}