J.,
I also agree with the other moms...there really is no reason to tell your daughter. I am all for being open and honest, but this is of no concern to your daughter. I would just like to add that if she does ask use discretion in your explanation, i.e-if he cheated or was beating your sister-things like that...kids do not need to know these things at the young age of 4. I have twins that are almost 7 and we have just gone through a similar experience-however their "uncle" was very much a part of their daily life. They have not asked many questions at all...every once in a while they may mention his name, but that is about it. I think that we as adults tend to make a bigger deal about letting our kids in on things that they shouldn't be dealing with. Good luck!