In a recent news post at Starwars.com the topic of Celebration 4 was brought up. Here's the line from that article...

"We look forward to bringing the same experience to our members with the two Star Wars television series in development, an anticipated Celebration IV event, next-generation Star Wars video games, special theatrical and DVD releases and whatever else the future has in store."

"...an anticipated Celebration IV event..."

The fact that this was even brought up on the official site pretty much confirms it for me. In the past Lucasfilm has been very tight lipped about all their projects until officially announced. This little quip is a very positive sign that things are coming together on their end in the planning stages.

Now all we need is an official announcement ( San Diego Comic Con? ) so we all know when and where to meet up for another great Star Wars Celebration!