The short answer is, there is no usable RAM there so you can't use any. If you have an MDA video card, it has only 4096 bytes of video memory, and for 80x25 screen, it uses 4000 bytes already, so there is 96 bytes that are not visible. If you need more memory then you need to add memory, either just plain memory cards that can provide memory, or an EMS card which can add paged memory.
The 8086 can only address 1024 kilobytes, and when making the 5150 PC, IBM decided to split that so that up to 640 kB of RAM is possible and the rest of the memory, 384 kB is reserved for system use and expansion cards.
Some non-IBM systems could allow to have more memory and start the expansion memory area later, but if they were compatible with PC graphics there could only be up to 64k or 96k more before the start of video memory area.
Typically there is only up to 640 kilobytes, either all of it on the motherboard or in some cases some of it in the expansion slots. You can't even assume you have the maximum amount installed.
So, assuming there is a video card installed, you can use it for reading and writing, but it would be slower to read and write than main memory and everything you store there will visible on screen, as that is the purpose of video memory. Only later cards such a Hercules, EGA and VGA had more memory than needed to show on one page.
So if there is no special RAM cards that add general memory to the 384k reserved area, then there is no memory.
Typically, you would not be able to use the video memory in any meaningful way to store arbitrary data, and that memory would typically not be visible by DOS, not without tricks.