A four year old's understanding of death

I have always been fairly straight forward with our little one about death.  Trying to explain things at her level but being honest about it. (The cat didn't go to someone else's house like nonna says, he probably died)  Maybe it is a bit harsh, but I lost my baby brother when I was five and was not told anything but the truth about it and I was much better off for it.
Last year my husband's grandfather of nearly 91 died after many long years of battling prostate cancer.  We went to the funeral (that time my daughter was 3) and I tried to explain that we were sending nonno to be with baby Jesus.  She took that as good, and made the connection with the fact that he was no longer at his house when we went to visit.  Later on during a thunderstorm my mother in law said to her "ah that's nonno shelling the nuts!"  She repeated this line a couple of times, but now doesn't really believe it.
At the beginning of the week my in laws went to the cemetery to take flowers to Nonno Peppone's grave for the one year anniversary of his death.  When my daughter asked me where they were going, and I told her that they were taking flowers to nonno because it had been one year since he'd died she said in a very happy voice "IS HE COMING BACK!?"  It was such a sweet thought that nonno had gone to visit Jesus, but was now coming back to be with us.  Unfortunately, I had to tell her that no he wasn't coming back anymore.  She was a bit bummed about it, but let it go pretty quickly.
That was almost as cute as my sister (age 3 at the time) asking my mom if there were phones in heaven so she could call our little brother in there.