I get the sense that what you are really asking is, “is my 4 year-old ready to attend and understand a funeral”, “Will he get scared from seeing an open casket”. The answer to these questions is, “depends on your child”. Most kids this age will not understand why everyone is so sad, or why there is a person lying in the coffin. If your child is old enough to understand death, then maybe he will understand a little. He will probably not remember the funeral when he is older.
I think there’s a more important question to be asking, “Will your 4 year-old allow you to fully devote yourself to the mourning process at the funeral”. A funeral is an important time for those whose lives were touched by the deceased. It is a time to reflect on your loved one’s life and to encourage and bond with those left behind.
Having recently attended my wife’s grandfather’s funeral, I am very glad we left our then 3 year-old home with a sitter. The main reason for this is that he would have whined and fidgeted through the whole long half day of service, burial, and luncheon. Add to this the stress of a 4 year-old pulling on your leg the whole time complaining, “I’m firsty”, “my shoes hort”, “I want to jump on those rocks”. My wife and I would have been chasing him around the entire cemetery all day (just like at home!). He would have ruined our day of saying good bye. We were able to concentrate on our relationships with our loved ones, cry with them, reminisce with them.
Our son is now a typical 4 year-old and he is not yet ready to act appropriately at this type of function. He gets to come to baseball games, movies, but not funerals. I don’t think he “missed out”, he was too young to understand. My 8 year old daughter does not remember very significant events of her life that happened 4 years ago. She did come to the funeral and it was a wonderful experience for her.