There are a lot of theories going around online as to what the fourth and final disaster is going to be. I actually read somewhere online that the fourth book is going to be named Mission 39, although I haven't seen that confirmed.
There's one theory that each disaster is going to be 32 years apart. That would place the fourth disaster happening in 2008. In 2008, the Clue Hunt was going on. It's possible that the final disaster could involve the serum (please no), or the Outcast could create his own disaster that would pit all of the Cahills against each other once again.
As for Mission Hurricane, the third book, one possibility is that the disaster won't have anything to do with an actual hurricane, but a mistake that was made in the aftermath of the hurricane. Didn't the Outcast say something at one point about always being able to rely on human stupidity? I'm pretty sure that he's not going the whole natural-disaster weather machine route. He seems like too cool of a villain. Let's face it, V1 was a nutjob...
I don't know. There are a number of different ways that that could go. I think that we'll be able to formulate better theories once we have more information on Mission Hurricane. What do you think?