L.S.
It is completely normal for kids to get fixated for a little while, especially on something as profound as death and heaven.
Maybe just reminders that while it is appropriate to talk about it, that there is a time and place for it, and just be honest and asnwer whatever questions he comes up with in a way a 4 year old will understand.
I was 4 when my father died, and I was very upset from the viewing... I didn't understand why my Daddy was in a box, and would not go to the funeral. I was fixated for about 6 months or so before I found something else to be fixated on... which at the time happened to be cutting my own hair! :-)
Death is horrible, and I have dealt with my fair share of it in my life... the one thing to always remember is that we are still alive!
Good luck, I think as long as he is not being morbid or making fun of it, it is just a normal phase... if things don't change in the near future, maybe talk to a pediatric counselor or a greivance counselor who might be able to give you a little more insight.