K.P.
You know your family and your situation best, so take your cues from your daughter. The odds are very good that she would describe your household as a tense and unhappy place to be, so you may want to focus on the fact some of the "bad or scary" things will stop and the "good things" will stay.
Be honest and clear w/o giving her more info than she needs to know. Don't treat it as a "trauma" b/c then she's going to think she's being "traumatized". When I was working with families, I would talk with very young children about the fact that there are all kinds of families and we would draw a family "web". I would have the children draw a picture of each family member on a notecard (one for each person) and then lay them on the table. Using yarn or string, tell the "story" of your family connecting the people.
"Mommy and Daddy met in college. We liked spending time together and wanted to spend all of our time together, so we got married (tape yarn connecting the two of you). Then, we decided to have children and you were born! We both love you very much (piece of yarn b/w her and each of you- separate pieces so she's "connected to both"). Then we loved being parents so much we had your little brother (more yarn). But Mommy and Daddy had a hard time using nice words with eachother and are nicer to eachother when we don't spend all of our time together, so we decided that Daddy should live ______ and that we shouldn't be married anymore (take away the yarn b/w you two). But look, just because we aren't married anymore doesn't mean that we aren't a family anymore. We're just a different kind of family".
It sounds silly, but it is a very tangible and visual way to reassure young children that their parents are still connected to them and to eachother as parents. Answer her questions as they come up, but don't dwell on the topic either.
If she really isn't dealing well with the situation, get her a therapist!