NAND allows only page access… this means that you cannot address this part of SSD drive manually…
I mean creating a partition and to store some files there… (I think you mean exactly this..)
The 4GB are used to for file access or system boot…as far as I know its used for virtual memory. Virtual memory combines your computer’s RAM with temporary space on your hard disk When RAM runs low, virtual memory moves data from RAM to a space called a paging file stored on HDD.
However, usually the computer can read data from RAM much faster than from a HDD but due to the 4GB on SSD drive this sharing would be much faster.