I think the Challenger disaster would rank second for me, not fourth. And Princess Diana’s funeral is there, but her wedding isn’t there. I also remember the night she died and being gathered around the television in the dorm much more than her funeral.
For me Challenger would be #1. Maybe I was too cynical at 25 when 9/11 happened for it to have the same impact on me, and maybe 24 hour news coverage has deadened the impact of anything, but 10 year old me watched the Challenger go up and everything changed.