int ButtonStepForward = 2;
int ButtonStepReverse = 3;
int ButtonServoMetal = 4;
int ButtonServoNoMetal = 5;

void setup() {
  Serial.begin(38400);
  pinMode(ButtonStepForward,INPUT);
  pinMode(ButtonStepReverse,INPUT);
  pinMode(ButtonServoMetal,INPUT);
  pinMode(ButtonServoNoMetal,INPUT);
}

void loop() {
  if(!digitalRead(ButtonStepForward)){
    delay(100);
    Serial.print('a');
  } else if(!digitalRead(ButtonStepReverse)){
    delay(100);
    Serial.print('b');
  } else if(digitalRead(ButtonServoMetal)){
    delay(250);
    Serial.print('c');
  } else if(digitalRead(ButtonServoNoMetal)){
    delay(250);
    Serial.print('d');
  }
}