Nearly 4yr old being mean at school

[Kataalam]

This is my first mumsnet post!!
My 3yr old daughter, who will be 4 in may, has always been a lovely child, a little feisty at times, but generally well behaved.
However, in the last few weeks and days, her behaviour seems to be changing. She has been mean to my mum (who looks after her while I work) saying things like "I dont like you" and "I don't want you to be my grandmother anymore" - she says things like that to me as well, and I have generally just ignored it.
However, in last few days, I'm getting feedback from her pre-school that she is saying mean things to the other kids in her class, especially to the son of a good friend that my daughter has pretty much grown up with.
We are expecting twin boys to be born on Monday, and i wonder if she is in some way reacting to that, but at the same time, i hate the thought that my daughter might be upsetting other children. It's one thing if it's her family, but other children is a different matter. I know her teachers try to make the kids think about their feelings, and instead of just telling them off, try to explain why it isn't nice to see mean things, but when I try to explain, using the same vocab, she doesn't seem remotely bothered.
Does anyone have any experience with this sort of thing? I want to make the transition to having 2 more babies in the house as smooth as possible, but ag the same time, dont feel I can just let her behaviour slide.

I also have a 3 nearly 4 year old DD (4 next week) who has discovered a new talent for being mean, although she doesnt have the excuse of 2 new babies about to arrive!

DD has started to be a bit slappy and shouty with both DS and DD2 (5 and 20 months). After trying to ignore her with limited success I am now using time out which seems to be more successful. As soon as she slaps or shouts or name calls I sit her on a chair or the sofa or kitchen stool and say 'that was unkind, sit here until I ask you to move'. I then stay nearby but make sure I talk to DS or DD2 or get on with something else. Then I go back and say 'thanks for sitting nicely, go and play again'. I find if I remove her swiftly enough then it doesnt escalate. If you can then it would be good to get your mum doing the same with her.

I dont really think that she has any understanding of how being mean makes someone feel, she just wants what she wants and will do whatever she can to get it. If it's someone particular at preschool then I would ask the teachers to keep them apart where they can. There may just be a personality clash.

Also, I am trying really hard to tell DD when she's being lovely, and praise like mad if she shows any sign of sharing or being kind to someone else. Have tried this for about 2 weeks and for the last few days she's been much improved.

Hope all goes well on Monday

Awwwwwwww. Should think it is a reaction to the babies coming- good luck with that!!!

She is still very young and I would be very gentle in steering her towards being nicer-she may not be able to help herself especially if it isn't really about being mean but feeling vulnerable. I would reassure about how she will be loved etc as well as new babies and talk generally about feelings and being kind to other children, but I would give her a fairly wide berth.