Only two seasons have had a final 5 -- Cook Islands and Fiji. So a final four is the norm, not an exception. Nothing odd here.

It will likely still be a final 3 for final tribal. Jeff has said that can't really ever go back to a final 2 now. Plus, having just two people at the end was getting really boring because it would be so predictable who was going to win. In China, they just did a reward challenge and an immunity challenge to make up for not having two immunity challenges like the old finale days. It will all work out, kids.
Once again, as I explained a few posts ago, the point I'm trying to make here is that this means someone is removed from the game in either Episode 12 or 13. If 1 person was eliminated in both of those episodes, there would be 5 people in the Finale, but apparently there's only 4 people in the Finale, so clearly someone must have been removed from the game. Get it?

Also, although it's clear that the show INTENDED for there to be a Final 3 this season, that doesn't mean that it happens. Sometimes unexpected things happen, like players being removed from the game, that force them to change their plans. Granted, they could still do a Final 3 even with 4 people in the Finale, like they did in China, but it means that they would have had to drop one of the immunity challenges and add an extra reward challenge so they would have a challenge to show for the first half of the Finale. They may have decided that it was easier to have a Final 2 than to come up with a new reward challenge at the last minute.