Came here to say something similar: I didn't realize how rare this was until this thread, but as I kid I knew not only four of my great-grandparents, but even a great-great-grandmother. She died at 96 years old, but I was old enough to hang out with her a few times, and remember her.

My great-grandfather is still alive and turned 100 last year. Since I'm already much older than any of my progenitors when they had their children, even if I live to his age it is unlikely I'll get to spend time with my own great-great-grandchildren.