Is there a way to repartition the internal 4GB of my htc sensation? Currently, only 1GB is available for use (/data) The rest is being allocated to /system, /cache, etc.
I don't know for sure, but think changing internal partition layout can only be done on special development devices with the security turned off (called s-off).
The problem of running out of space on the internal data partition has been solved by many custom ROMs that can be installed after using the official HTC unlock process, however.
These custom ROMs have various methods of supporting putting apps on the SD card (in a more seamless way than standard Android) or putting even more of the data partition storage requirements on to it (although this often requires formatting the SD card to the EXT file system and sometimes even multiple partitions).
Make sure you go through your app list in Settings first, moving anything you can to SD card, by the standard Android method. This is often enough and easier than installing a custom ROM with a2sd(+)/d2w support.
if you were to have a custom recovery installed onto your device, it may have options to partition the internal storage.
At least, the SGS2 CWM Recovery does.