In need of serious help and advice

My 4 year old is very strong willed. She is also very independent. I’m at my wits end with her. I love her to death but her attitude and defiance is getting to me. She has the attitude of a hormonal teenager. She doesn’t listen and I find myself repeating things 4 or 5 times. Which then leads to her getting in trouble and crying and whining for a good 10-15 minutes. I’m tired. Not to mention she has a 6 week old sister who needs a lot of attention. Her dad (my husband) has been home for paternity leave and I’m not sure if that’s why she’s acting out. She behaves well when with one of us, she listens and is so sweet. But when it’s both of us, it’s like World War 3 in our house. She talks back, she spits, she gives attitude. It’s horrible. I just don’t know what to do anymore. We’ve tried talking to her, we’ve tried time outs, we’ve tried taking things away, we’ve tried spanking as a last resort method, but nothing works!! She screams and cries, which we ignore. We try praising the good behavior as much as possible. I just don’t know what to do anymore. She’s like a teenager already!