^ You could be right. But Halle would cost a fortune to get back and Rogue lost her powers in X3 (not to mention that Anna is tied up with True Blood). I don't see Ashmore or Cudmore putting a lot of bums on seats and Cumming has made clear his hatred of the make up process.
My main reason for not expecting a sequel is that studios tend to think audiences have the attention span of a gnat and find it easier to reboot than make sequels. The more successful Spider-man movies are being rebooted after a shorter gap and with a more open-ended 3rd movie. Superman, Hulk, Punisher, Ghost Rider and - apparently - Fantastic Four and Daredevil are also being rebooted, after shorter gaps than there's been since X3. I'd be surprised if X4 bucked the trend.
of course, we're broadly in agreement here anyway!