Bluetooth module using 4 motors robot in Arduino uno

#include <Servo.h>

Servo servo_motors[4];

int motor_pins[4] = {9, 10, 11, 12};

int speed = 0;

void setup() {
  Serial.begin(9600);
  for (int i=0; i<4; i++) {
    servo_motors[i].attach(motor_pins[i]);
  }
}

void loop() {
  if (Serial.available()) {
    switch (Serial.read()) {
      case 'a':
        servo_motors[0].write(180);
        servo_motors[1].write(0);
        servo_motors[2].write(180);
        servo_motors[3].write(0);
        break;
      case 'd':
        servo_motors[0].write(0);
        servo_motors[1].write(180);
        servo_motors[2].write(0);
        servo_motors[3