My partner's 4 year old daughter lives with us. I look after her more than my partner. She is generally well behaved, polite little girl and we have lots of fun. As soon as Daddy comes home she turns into this demanding whiny little brat. I can not stand it! I think it is because Daddy is much softer than I am so she knows she can be more demanding and play him up. Is this learned behavior? Why is she so well behaved for me?
Children have a very sophisticated sense of what works to get what they want. Your partner's daughter isn't a whiny brat around you, because you don't respond well to that behavior, but apparently your partner gives in easily to that kind of behavior.
My own daughter has two personalities she puts on for me and my wife. For me, she is sweet and sugary, because she knows I can be charmed, but refuse to give into whining. For my wife, she is whiny, because my wife can't be charmed, but gets worn down by whining.
The upshot is the behavior won't change unless your partner commits to not giving into it.
Yup, agreeing with everyone else, your kid whines for Daddy, because she has empirically learned that it gets her what she wants from him, not from you. Your way is better, you are doing more child care, he needs to get on board, and not give in. He needs to be saying "I know you are disappointed, (acknowledging the child's feelings, helping the child to name her own feelings is really helpful) but I'm not giving in".
I have worked in childcare for many years, and it is my experience that kids will behave much better for caretakers other than their parents (this is true with young elementary school kids as well). It is possible that since "daddy" is a biological parent and (I assume) you came into the picture sometime after she was born, she views you in this way.
I also could be completely wrong, seeing as I don't know your situation. I recognize how you might take offense to this and I assure you I mean none.