int a = 0;
void setup() {
 Serial.begin(9600);
 pinMode(3, INPUT);
}
void loop() {
    if(Serial.available() > 0){
      a = Serial.parseInt();
      Serial.println(a);

     if(a != 0){ switch (a){
        case 1: digitalWrite(4,1); break;
        case 2: digitalWrite(5,1); break;
        case 3: digitalWrite(6,1); break;
        case 4: digitalWrite(7,1); break;
        default : digitalWrite(4,0);digitalWrite(5,0);
        digitalWrite(6,0);digitalWrite(7,0); a = 0;break;
      }}
    }
}