At least you would have a shot if your soon to be ex were dead. A child as young as your can be told that mommy has gone to be with god.
You might want to consider telling your daughter that mommy is far away for what ever reason that you can come upon with when her question is elaborated on. As in, when is mommy coming back?
If the impending divorce is a result of addiction, consider telling your daughter that mommy is sick and cannot come home now.
It is a tough place that you are in. My heart goes out to you and your child.
Jay