#define Ap 20
#define Am 21 
#define Bp 19
#define Bm 18

void setup() {
  pinMode(Ap, OUTPUT);
  pinMode(Am, OUTPUT);
  pinMode(Bp, OUTPUT);
  pinMode(Bm, OUTPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(100); // 1
  digitalWrite(Bm, LOW); digitalWrite(Ap, HIGH);
  delay(100);
  digitalWrite(Ap, LOW); digitalWrite(Bp, HIGH);
  delay(100); // 2
  digitalWrite(Bp, LOW); digitalWrite(Am, HIGH);
  delay(100);
  digitalWrite(Am, LOW); digitalWrite(Bm, HIGH);
}