Android 4.2.1 - Operating System Storage is occupying too much space
That device is partitioned very oddly.  It looks like they took 4GB and left it to device storage, and then the other 12GB are partitioned for an internal SD card.

That's about the correct size that the Android system would be using - but what is troubling is that they only left ~1GB or so for user-installable applications.  I doubt there are any ROMs or recoveries available for your device, so I think the best solution would be to move as many applications as you can over to your SD card, therefore freeing up space to download more (and then subsequently move them as well).