So very sorry !
As a parent educator, I would rec that you share your feelings of grief and keep the explanation tailored to a four year old.
When my daughter was 4, I had to tell her that an Aunt passed away. I told her that I was very, very sad. I told her that the tears were streaming down my face because I loved her very much ! I told her that Auntie's body gave out and she was now in heaven. Then, I reassured her that we would say prayers every day and that God would be taking care of her now.
Because a 4 yr old might think that death is reversible, I told my daughter about a soda can. When a can of soda is opened, it's like the soul goes to heaven and Auntie would be living w/ God now. We still have the can of soda--as the wonderful memories of Auntie.
The next day, I gave her more reassurance that she was ok (us, too) and healthy & we would live a very long time. Sooner or later, a child at that age will begin to wonder if she or another family member will die. It's developmental.
It is important that you do tell him.
I would ask the child librarian to put a couple bks aside for you. Skim through a few of them and decide which 1 you want to read to him.
At that time, I had the book, "When a Pet Dies" by Mr. Rogers in the house.
Also, my daughter would play dolls and the Auntie would die. That is 1 of the ways she was trying to understand...through play.
Do tell him, keep it very simple and offer reassurance when you see it's needed.
Again, so very sorry for your loss!