A.H.
Death is a very normal part of life. It is greatly feared as children are afraid of being alone (or lost). This is normal for the age, from much of what I have encountered and my friends have had happen.
However, I follow a different faith and belief: I feel that we are not gone... I feel that my grandmother that has passed is still there, watching over me and protecting me by helping me see things so I do not get hurt (such as in traffic or walking and avoiding falls.) I find that this theory has helped my friends children. When my son begins more talk of death (he is only mentioned it as "I died!" joking and in play), I will also tell him the same.
I think if I were to tell him we will always be here, and if by some unfortunate mistake out of my control, it would shatter him? Even for a child of almost four, my son has a memory that just amazes me~ I think I would say something like "Mommy and Daddy will always be with you - even if we are not with you in person, because we are in your heart. We love you so very much nothing could break our bond! You may not hear us, but you will know it is us." But as this has not happened yet, my speech is obviously not up to par! :)
I think if your little one is in a stage of worry, you could always allow him to contact loved ones via computer (email/ webcam) or cell phones (obviously when it is convenient!). And just keep on loving your child as this will always overcome most things! ;)