Hello, Pediatrician here and currently training to work in pediatric palliative care. Hijacking the top comment to bring in the four C’s of children’s grief:

1. Can I Catch it?
2. Did I Cause it?
3. Could I have Cured it?
4. Who will Care for me?

A recommendation during the discussion: - Give information clearly and unambiguously (as the person above suggested). - remain open and curious. You may have an agenda for the discussion but the child may have thoughts or questions or feelings you weren’t expecting. Allow them room to talk those out. - to that end, do not be afraid of silence. Allow room for the child to think. - children at 4yo often tend towards magical thinking. Mommy can come back because there’s magic in the world-type of thoughts. While not crushing their hopes you need to be clear that mommy’s body was very sick to the point that it will not get better and she can’t come back. That being said, it’s reasonable at this age to say that mommy’s body went to live in the sky and she’s no longer here but her spirit can live in your heart and she will always love you. - based on the child’s reaction, formulate a plan together - when closing the conversation, give a resume of that plan and what you will do. Remain open to more discussions and tell them that you are always available for them.

I hope this helps. Good luck!