Even the screenwriters have no scenario in mind, see this interview with co-writer Ehren Kruger:
Q: So you’re shifting gears in this film. You’ve got new human
characters. But you never actually mention in the film, maybe in the
script I don’t know, what happened to Sam. Was there a line of
dialogue that got cut where you addressed that?
A: I don’t think it ever made the script. I think we talked about at
one point referring to the “Witwicky Incident,” but it never made it,
Q: Do you have any ideas about what he’s doing right now? Because he
seemed really eager to be part of the Autobot experience in the
previous film, so I wonder how he feels about all this.
A: Well, the reason we ended up not wanting to talk about that too
much was that we liked the idea of venturing into this franchise from
the perspective of humans on the outside, like the audience, that have
just sort of watched these events unfold through the news and through
television, and who didn’t know any of the characters or experience
the first three things. So they’re coming into this with entirely
fresh eyes, without an awareness of things that went on in the
previous three films, and well, we liked the purity of that. So we
didn’t want [get hung up] with the characters who had been in the
So until someone decides to do some kind of tie-in media or a new movie that addresses it, I don't think you'll find any kind of remotely "official" answer, only fan speculation.