Helllllpppp 4 year old

My 4 year old daughter is going to give me a run for my money for sure!!! She is so mean she tests my limits on a daily basis if I say no she does it in front of my face she back talks me and I do t know why she acts this way she acts so horrible alot of times she is even in a thing where she does not sleep at night at all out during the day I also have a 1 month old and between the two of them I feel crazy I don't know what to do anymore