Default [Q] Storage

Out of the advertised 16, I get 11 GB and change.

I know the rest is used "internally", but can anyone clarify - what exactly is using these 4.something GB? A factory ROM, radio partitions? What else?

Is there any ROM or hack to "repartition" this and free the space from unneeded stuff or is it too dangerous and pointless anyway cause you can always use OTG or SD-Card?

Not really a pressing "help me" question, but applied curiosity.