Arduino Serial Control - Strings?

Using Arduino Programming Questions
14

Ok guys, my friend has helped me and made this little code:

#include <stdio.h>

#define LED 13

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  if (Serial.available() == 4) {
    char command[5];

    for (int i = 0; i < 4; i++) command[i] = Serial.read();
    command[4] = '\0';

    Serial.println(command);
    
    if (strcmp(command, "AAAA") == 0) {
      digitalWrite(LED, HIGH);
      Serial.println("LED13 is ON");
    } else if (strcmp(command, "BBBB") == 0) {
      digitalWrite(LED, LOW);
      Serial.println("LED13 is OFF");
    }
  } else {
    Serial.flush();
  }
}

Everything works, except for one little and very important thing:
If I send AAAA, it will print AAAA and do the action. However If I send AAAAA, it will make everything the same way and print only AAAA. But when I send BBBB (for example), it will print ABBB. In my words, It is "saving" a character. And this must not happen since the router will be sending "random" stuff trought serial that must be ignored and must not mess with the commands that I send.

Anyone knows how to solve this? Thanks!