Samsung has issued a slightly vague response to that kicking it took on the BBC's Watchdog last night (above), claiming it's the "innovative and unique features" that hoover up so much of the phone's internal storage space. However, it is looking at, perhaps, optimising things to free up a few more bytes for use.
Here's the full statement from Samsung. You can't get much more vague a promise than saying you're "reviewing the possibility" of doing something:
"As one of the most advanced smartphones on the market, the Samsung Galaxy S4 uses part of its internal memory to bring our customers its innovative and unique features. We appreciate this issue being raised and we will improve our communications. Also, we are reviewing the possibility to secure more memory space through further software optimisation. Samsung is committed to listening to our customers and responding to their needs as part of our innovation process."