I've been commenting this all over these forums but my personal theory is that they have some kind of inside info, but no title or details. These same guys had a posting for TS4 Cats and Dogs before it was officially announced but called it "Pets" like the TS3 equivalent title. Apparently, they also did the same thing for "City Living" but I haven't been able to find a source for this claim.
They might very well know that the next expansion is a TS4 equivalent of Island Paradise in TS3, but when they originally announced it, they took the name and details from this forum. They probably thought it was an official leak because it was on a sims forum and didn't bother to scope it out; combining it with their posting just to have something to say. When it became common knowledge that it was just a fan desire/speculation post (and someone from here messaged instant gaming and let them know) they changed the title to "Island Paradise". So, they obviously still think it's some kind if tropical EP, and they've been right before.
Doesn't make it concrete proof, and I could be totally wrong, but this is just what it feels like happened to me.
