void setup(){
for(int x=4; x<=13; x++){
  pinMode(x, OUTPUT);
}
}

void loop(){
for(int x=4; x<=13; x++){
  digitalWrite(x,HIGH);
  delay(300);
  digitalWrite(x,LOW);
}
for(int x=13; x>=4; x--){
  digitalWrite(x,HIGH);
  delay(300);
  digitalWrite(x,LOW);
}
}