How to convert String to byte array
Why? And how do I receive desired output? getBytes docs


The String member function getBytes() will null-terminate the string. You must add space for that. 

  byte user1[17], user2[17], pass1[17], pass2[17];
  ...
  user1str.getBytes(user1, sizeof(user1));
  ...
  Serial.print((char*) user1);


That should do the trick.