const byte PinLeds [] = { 12, 11, 10, 9 };
const int  Nleds      = sizeof(PinLeds);

bool       ledState [Nleds];  // default false (off)

void setup() {
  Serial.begin(115200);
  Serial.println ("rdy");
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read ();
    int  i = c - '0';

    if (i < Nleds)  {
      if (ledState [i])  {    // if on
        ledState [i] = false;
        digitalWrite (PinLeds [i], LOW);  // off
      }
      else {
        ledState [i] = true;
        digitalWrite (PinLeds [i], HIGH);  // on
      }
    }
  }
}