The Meaning of Forever

What exactly does "forever" mean? How does a 4 year old interpret the word "forever"? Alexis asks questions all the time about my dad, but this one seemed to really be meaningful. Alexis was reading a book about Mickey Mouse at the beach in the car to herself. Then she says, "I want to go back to the beach." I said, "me too, I love the beach". But that wasn't quite what she meant and she elaborated, "No, I want to go back to the beach with Opa". Now, that was exactly a year and a half ago, which made her 2 1/2 at the time. But she has very clear memories of being there. I asked her what she liked about being at the beach. She said, "building sand castle birthday cakes with Opa and then the waves washed them away and floating in the ocean and tasting the salty water". I told her that we can go back to the beach and think of Opa when we are there, but he cannot come with us. Then through tightly gritted teeth, she says quite angrily, "why does he have to be with God forever?" What a question for a 4 year old. Does she understand what forever means? I didn't think she did up to this moment. She would ask often, "when is he coming back from God" or "when he's done with God, I'll see him" or various things like that, but never indicated that she knew it was forever. She used to accept my answers so easily that I envied her ability to be "okay" with the facts and not have the ability to analyze and obsess, but as she is getting older, it isn't pacifying her anymore. After a few back and forth comments, she didn't want to talk about it anymore and the conversation was over, but a very enlightening and difficult conversation for the 9am drive to school.