While Beastly Kingdom was definitely more than a rumor at its time, what the one member was told by her daughter about a new park is mostly likely false. The rumor of a 5th park has been circulating for awhile...and every time the rumor resurfaces, I always hear of more people who know CMs who have "inside info" as to what this mystery park shall be. And every time, it's something different. IF a 5th park were to be opened, I highly doubt they'd be sharing the news with all the CMs until it was official....especially considering many of those CMs aren't there for long term.
On another board I'm on, Disney related, I DO know someone who is higher up in the Disney company (not merely someone who works at one of the parks). We've discussed this "5th park" rumor with him several times, he's always debunked it. Now, it could be he knows more than he's letting on, but he did bring up a good point in that parks take MANY years of planning and then another many years to build. One ride alone can take 2 years to build. So something tells me that a 5th park is not going to be talked about in meetings and such with regular CMs until this has all been planned out and officially decided on.