How to talk about dead to a child? If there's a way...

I have a 4 year old. This morning I read an status update in Facebook of a dear friend of mine and she was asking this same question. It's hard to face the loss of a loved one but how do you explain that to a child?? How do you tell them that this person is no longer here with us? That your doggie or cat went to dog/cat heaven? How do you tell them? At a given point of our lives we may or may not have to face this situation and how to handle it? My thoughts are with her and her family...