M.B.
My 3 year old son's become increasingly interested in death since his grandmother's cat recently died...3 year olds and 4 year olds can be pretty different, but in my son's case, he's playing the same sorts of games and asking the same questions, I think, because he's trying to understand the concept. Fair enough- it's hard for a lot of adults to grasp the concept.
At first I changed the subject whenever it came up, not really wanting to delve into the topic so soon in his life, but I think that, in doing so, I made it even more interesting/mysterious. Now when it comes up I tell him that sometimes our bodies become too old, tired or sick to take care of us, and that that's when our body dies. And I leave it at that. If he's wanting to play a death game I just point out that his body is healthy and strong and that this just isn't part of our day.
I know, 3- and 4- year olds can be really different, so I don't know if this will help at all. Thought I'd toss my 2 cents in, though, in case it would work for you. : )
M.