R.M.
"I don't understand why she would want to create a negative environment between her parents..."
I'm sorry if my response is going to sound harsh, but you need to understand something: this child is only FOUR years old. Don't ascribe motives to her that aren't even within the capacity of her not-yet-fully-developed brain.
She is still a baby. If a four year old is doing the type of things that you are describing, the fault lies firmly with her PARENTS. Don't blame a four year old for the results of her parents' behavior and parenting. Don't blame a little girl for "pushing everyone's buttons." You guys are the adults.
Any four year old who is acting out severely is not being properly parented. Period. Both sets of parents need to reevaluate how they are parenting, and make some big changes, fast. By the way, the best way to teach morals and values is to model it. If you are sure she is not hearing all this "adult language" from you, then she must be hearing it at her mom's house. She is not picking it up all by herself. If she really says she wants to stay with you forever and not go back to her mom's house, then there is something negative going on over there.
If you need specific parenting advice, you can get it on this site.