Looping

int Fdelay = 250;
int Bdelay = 125;

void setup() {

  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
}

void loop() {


  for( int z = 3; z < 7; z++){
    digitalWrite(z, HIGH);
    delay(Fdelay);
    digitalWrite(z, LOW);

    if( z >= 6 ){
      for (int i = 6; i >= 3; i--) {
            digitalWrite(i, HIGH);
           delay(Bdelay);
           digitalWrite(i, LOW);
      }
    }
  }



}