Try using the following code you need to use String and concatenate that

    String rcev_data="";
bool chk=false;
\#define output 9

\#define output2 10

void setup() 
{
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(output, OUTPUT);
  pinMode(output2, OUTPUT);
}

void loop() 
{
  // put your main code here, to run repeatedly:
  while(Serial.available())
  {
    char rxx = Serial.read();
    recv.concat(rxx);
    delay(10);
  }
  
  if (recv == "QWER")
    chk=1;
  else
    chk=0;

  if (chk)
  {
    digitalWrite(output,HIGH);
    Serial.println("ON");
    delay(3000);
    chk=0;
  }

  else
  {
    digitalWrite(output,LOW);
  }
  recv="";
}

Try using the following code you need to use String and concatenate that

    String rcev_data="";
bool chk=false;
\#define output 9

\#define output2 10

void setup() 
{
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(output, OUTPUT);
  pinMode(output2, OUTPUT);
}

void loop() 
{
  // put your main code here, to run repeatedly:
  while(Serial.available())
  {
    char rxx = Serial.read();
    recv.concat(rxx);
    delay(10);
    if (recv == "QWER")
    {
       chk=1;
       break;
     }
    else
     chk=0;
 }
  
  

  if (chk)
  {
    digitalWrite(output,HIGH);
    Serial.println("ON");
    delay(3000);
    chk=0;
  }

  else
  {
    digitalWrite(output,LOW);
  }
  recv="";
}