Most people, including children don't like to be told they are wrong. If you start with "That's wrong ...", it might immediately cause her to react emotionally to the task. At 4, her mental management skills are not fully developed and she would find it difficult to manage her emotions.
Perhaps you can try a different approach. You might praise her for her effort (not work) and then ask her if she is happy with her work. You could also ask her if she would like to know how to make her work even better. Or say, Mummy writes her "O" this way, how is that the same or different than the way you write? Let's do it together, I will show you how to do it.