M.S.
I have worked as a counselor with many children of divorce, and those that seem to adjust the best are those whose parents explained to them that they are not going to be married anymore- but that they are both still the child's parents and love them. It is also necessary to emphasize that this is not the child's fault, and at age 4 - developmentally, the child will need reminders and repetition of this. Stay child focused. Remember the adults are the ones with all the choices, and the children do not have any. So the more input the child is allowed - the better. Please do not ask the child who she wants to live with....this is extremely damaging to the child, and custody is an adult to adult decision.
Best wishes to you in the most difficult time. You and your spouse may consider seeking co-parenting counseling sessions which focus on how to do this with the least amount of stress on the child. There is also a website called: nirvanamommas (I am not sure about the spelling).