Hi H. -
I am sorry to hear of your situation - it must be very hard.
I volunteered as a hospice volunteer for a few years and have found that, children are very aware of what's going on and that it's important to talk with them using honest but carefully chosen words to explain death and dying. The more a part of the life cycle it appears to them, and the more questions they get answered, the better they can adjust. One of your others responders is correct, adults can have a much harder time dealing with death than children.
As far as going into specifics about what happens when you die, this depends a lot on your spiritual and/or religious beliefs. In this regard I feel explaining what you think happens when someone dies in a pleasant way will probably be the most helpful because your child will be able to pick up on how your view death. You may also want to ask your child what he thinks happens, at age 4 they have picked up on a lot that we aren't always aware of (as I am sure you know!).
Recently my newly 3 year old son has had a LOT of questions about death lately and I explain to him that when we die our most special part of us, the part that has God in it, called our soul, goes all the way up to God, and he says, " welcome home!! I've missed you, now I will heal you!" and God helps that person heal and rest peacefully. I explain that our physical bodies stay here - kind-of like a shell. He always smiles when I tell him what will happen and seems satisfied.
I wish you all the best and will send prayers to you and your family during this difficult time.