It’s not a sign that you’ve done anything wrong, but it still needs to be dealt with.
Lots of children are rude, all will be rude at times (intentionally or not) and it needs to be handled accordingly.
So not intentionally rude I would say is when they, for example, point out something that as grown ups we know not to say. “That persons very fat” “thats a big spot on your face” “Your nose looks funny” “Grandads house smells weird”. You correct them each and every time, how would you like it if I said your hair looked daft today? Building empathy and consistency, they need to learn how to filter. Not punished, just discussed.
The rudeness in the form of no manners, answering back, cheekiness and such needs to be firm. As a teacher, I’ll be honest that I reinforce please and thank you with an iron fist (not literally of course. I’m no Miss Trunchable. But It’s my pet peeve).
To begin with in reception I absolutely covered the children in stickers when they remembered to say it (and not after being reminded or 2 minutes later when they realise they’ve missed the chance) and I frequently ignore children who don’t until they do, so If they wave their snack at me saying “open it Miss Bertie” I ignore but say to the class “goodness me I can hear someone speaking but sadly they’re not using their polite words so I don’t understand!” If they say “I want” I’ll likely say “pardon? I didn’t understand that”, until they remember. Such an important life skill and it’s not modelled for everyone, I work hard to model it everyday.
Answering back, arguing back, rude words to me results in consequences. As it’s reception, this is normally their free choice to choose what to play removed. We don’t do Time Outs, but if a child is rude to me or their friends I will choose where they go, normally a table by themselves with a puzzle or a book for a while. Children can join them but child needs to stay put (for a certain amount of time)
If they’re rude in a lesson, they may find themselves doing work in free play with me rather than choosing. I’ll move them from their friends on the carpet. I will call them out for it in front of the class, be form in explaining what they’re saying is not good and they won’t be chosen for special jobs or tasks.
I will just add that I’m not a dragon reception teacher! Starting school is hard, it’s a big adjustment and all the children from different environments are thrown into a big melting pot and will be teaching and learning from each other, and with that, as someone said above, comes standing their ground and standing on their own two feet which is a tricky thing for a child to do. They need lots of love and support through it.
And that brings us to the main point. Some children learn rude behaviour, some are reacting to the change, some are finding ways to cope with something they’re finding difficult, some are magnets to others who also find it funny. And it needs to be nipped before it escalates and the child needs support finding the right way to express themselves and learning to manage themselves when they’re cross, upset, tired, don’t get their way etc. That’s what being 4 is all about.
OP, I’m sure you are a wonderful parent so don’t beat yourself up about it. She’s 4. She’s gone through a big life change and the novelty has warn off. She will learn. Carry on teaching her the correct way to speak and act and react. Be consistent in enforcing manners firmly yet positively and use appropriate consequences for rudeness as necessary along with lots of praise for kindness and considerate acts. Be clear of what is acceptable, like I’m sure you always have been, and stick with it.