Darling, the fate of the astronaut in either case has nothing to do with it. The alpha centauri one is going to die before you get there, truly. But you cannot say he is "dead already" from your own reference frame. Well, you can on a
You can make the trip to the doomed astronaut, and you can watch him die through your telescope before you get there (however the hell that might look when you're moving at relativistic speeds) but there is absolutely no way for you to *prove* that he has already expired before you left, because there is no mechanism to check the fact.
"Oh well, but doesn't it just make sense anyway? I mean, he only had food for 4 years, and it took more than that for his message to reach us." Yes, I get what you're trying to say, but it's a philosophical point only, which is another way of saying it's no bloody point at all.