I have read 4 books about dying in hopes of finding the "perfect" book to help explain death to my 4 year old. I haven't found the "perfect" book, but this one came the closest and was the most helpful.
This book is actually part of a series of "Elf-Help Books" that feature a little Elf. What I liked about this book is that I was able to get one specifically about a grandparent dying, so it best fit my situation. Also each page has a clear topic title, so we could skip pages that I felt were too much for a 4 year old to understand. There are references to God, Heaven, a funeral, and Cemetery. There are photos of the elf throughout the book helping to show these different things.
If you go to Amazon and type in "Elf Help Books" you will see many titles dealing with death of other family members (death of a parent, a general death one), self esteem problems, moving, jealousy, fear, divorce, learning to be a friend, serious illnesses, being in the hospital, and others. I would recommend this series for a variety of situations that need to be explained to preschoolers.
I also read Up in Heaven by Emma Chichester Clark (a good book, but only deals with the loss of a pet), What's Heaven? by Maria Shriver, and I Miss You: A first look at death by Pat Thomas. But I liked "When your Grandparent Dies" the most out of all of these.