#include<SoftwareSerial.h>
SoftwareSerial serial (1, 2);
void setup()
{
   serial.begin(9600);

   
   pinMode(4,OUTPUT);
   
}

void loop()
{
  if (serial.available() >0)
  {
    char data = serial.read();
    if (data == 'a')
    {
      digitalWrite(4, HIGH);
    }
    else if (data =='b')
    {
      digitalWrite(4,LOW);
    }
  }
  
}