Did anything change in her life in February/March? Perhaps arrange a chat with her teacher to ask if anything has changed there if you can't think of anything, could be something quite small like where she's sitting or who she's with has changed.
I'd also wonder if she is using up so much effort being 'good as gold' everywhere else that she has nothing left at home. She's basically on her last nerve, and then you get the straw the breaks the camel's back. Perhaps try building in some recharge time immediately after school (or after wraparound care, if she goes). For some children that might be bouncing on the trampoline or swinging on the swings. For others, it might be cuddles and stories on the sofa. If her understanding is good, talk to her about what you want to try and why and ask her for ideas.
Once you have a sense of what helps her, you can try to use it when you see early signs she's struggling. It might be different for different situations – my similarly aged daughter is ordinarily a cuddles and stories sort of child when the day has been too much, but if she's getting frustrated, for example, a couple minutes on the trampoline to bounce the frustration out is much more effective.
Perhaps also think about keeping things low-key after school and at weekends for a bit. It might help to have as much as possible be predictable and easy for her whilst she's clearly struggling with something. Even stuff like food and clothes, I'd lean in a bit to her favourites and stuff that is easy to eat – things like cutting up food with knife and fork can still be pretty hard work at four. Again, I'd talk to her about it, would she like to go swimming this weekend (fun, but more challenging), or would she rather go to the park/whatever the low-key routine is again? If you need to do something out of routine or that she might find challenging, I'd try to build in recharge time before/after (and ideally with a plan for an emergency version during!).