I have a thread going in the GF IV forum which spins the idea that Tom Hagen did not die at all, but went underground to escape either

1) the Feds or
2) gangland enemies who wanted him dead.

Either one is certainly plausible. The Corleone Family would have the wherewithal to make Tom "disappear," set up a fake identity, etc etc. if for some reason he wanted to get out of sight of the Feds for a decade or so. In order to preserve this charade, Michael would tell anyone who asked about Tom, "Oh yeah, he died."

And as a formre consigliere and family lawyer, he would probably have been leaned on hard by the feds for information , creating the scenario of him going in the "official" Witness Protection Program.

so my GF IV script would feature a mid-60ish hair-dyed and sunglasses-wearing Tom Hagen reappearing to Michael in the mid-1980s after 20 years undercover.