My Sensation is running 2.3.4, why hasn't this updated? I've got a HTC Sensation 3G, and I'm waiting for the Android 4 Update.

I understand this update might not be available for my handset yet, but I checked my Android Version and I'm currently running 2.3.4.

Shouldn't this be 3.x at the moment?

I've checked for updates, but my device can't find any.
Android 3.x is reserved for Tablets, and so would never be running on an actual phone. As for 4.x, it's basically a waiting game. If there have been assurances from HTC that the update will eventually be available for your phone, you'll just have to wait until your phone tells you it's available.  The only way to speed things up would be to monitor the forums and see when others have gotten the same update.  Then you'll have a chance to download it yourself and tell your recovery to install it.  Even then its still a waiting game.