can arduino serial.read() values??

Using Arduino Programming Questions
13 
//encoder
volatile byte rpmcount;
unsigned int rpm;
unsigned long timeold;
//motor and keyin data
int cw = 7;
int pwm = 6;
int ledPin1 = 5;
int ledPin2 = 4;

void setup()
{
    Serial.begin(9600);
    Serial.print("BEGIN");
    pinMode(cw,OUTPUT);
    pinMode(pwm,OUTPUT);
    pinMode(ledPin1,OUTPUT);
    pinMode(ledPin2,OUTPUT);

//key in data
   digitalWrite(2, HIGH);
   attachInterrupt(0, rpm_fun, RISING);
   rpmcount = 0;
   rpm = 0;
   timeold = 0;
    
}


  
void loop()
{
  
  int inLetter;
  if(Serial.available())
  {
    inLetter = Serial.read();
    if(inLetter == 'C')
    {
     
      
      char buffer[] = {' ',' ',' ',' '}; // Receive up to 4 bytes
      while(!Serial.available());
      Serial.readBytesUntil('\n', buffer, 4);
      int incomingValue = atoi(buffer);
      Serial.println("R");
      digitalWrite(cw,HIGH);
      digitalWrite(ledPin1,HIGH);
      digitalWrite(ledPin2,LOW);
      analogWrite(pwm,incomingValue);
      //Serial.print(pwm);
      Serial.println(incomingValue);
      delay(500);
      
      
       
      
    }
   else if (inLetter == 'A')
   {
     char buffer[] = {' ',' ',' ',' '}; // Receive up to 4 bytes
      while(!Serial.available());
      Serial.readBytesUntil('\n', buffer, 4);
      int incomingValue = atoi(buffer);
     digitalWrite(cw,LOW);
     digitalWrite(ledPin1,LOW);
     digitalWrite(ledPin2,HIGH);
     analogWrite(pwm,incomingValue);
     //Serial.print(pwm);
     Serial.println("T");
     Serial.println(incomingValue);
     delay(500);
      }
      else 
      {
    char buffer[] = {' ',' ',' ',' '}; // Receive up to 4 bytes
      while(!Serial.available());
      Serial.readBytesUntil('\n', buffer, 4);
      int incomingValue = atoi(buffer);
     digitalWrite(cw,LOW);
     digitalWrite(ledPin1,LOW);
     digitalWrite(ledPin2,LOW);
     analogWrite(pwm,0);
     //Serial.print(pwm);
     Serial.println("T");
     Serial.println(incomingValue);
     delay(500);
      }
     }



  if(rpmcount >=20)
     {//Update RPM every 20 counts, increase this for better RPM resolution,
     //decrease for faster update
     float rpm = float (14*60)/(millis() - timeold)*rpmcount;
     timeold = millis();
     rpmcount = 0;
     Serial.println(rpm,4);
 }
 }
 void rpm_fun()
 {
   rpmcount++;
   //Each rotation, this interrupt function is run twice
 }

this program is functionable and i had test it.
i had solve the problems...thanks to all who guide and help me...this is the code used in my motor project, hope it can be improved and re-used to others...