I have heard a disturbing report that Natalie Portman was intentionally not invited to Celebration IV, and that Natalie felt hurt and snubbed because she was not invited. If this is so, it is an outrage, Natalie had just as much right to come to celebration as those actors that didn't play an important role like Natalie's. I would have loved to see Natalie at Celebration IV! I would have been just about as excited to see her as I was to see Carrie Fisher.
On the other hand, I have also heard a report that it is untrue that Natalie was snubbed and that she was just unavailable which I suppose does sound reasonable. However, it still does not clarify whither or not she was actually invited. Here are the possibilities that I see for this situation: she wanted to come but was not invited, she was neither available nor invited, or she was invited but unavailable.
Perhaps, we'll never know what happened. However, I do hope that I'll get to see her someday at a future Celebration.
Here is a link to a sight that declares the alleged snub untrue.