Question Hardware Reserved Ram

[GearDoGi]

Hi everyone. I'm new here but after reading many answers regarding my problem I've decided to tell you my story and questions.
So I have a very old PC.

-Computer-
Processor : AMD FX(tm)-6300 Six-Core Processor
Memory : 8Gb (2x 4Gb XMS3 DDR3 1333Mhz)
GPU : Nvidia GT 440 (1Gb)
MoBo : MSI a970-g43
Operating System : Ubuntu 20.04 LTS
(I use as a monitor a sony bravia 40 inch tv through HDMI cable and a small phillips monitro through vga cable at the same time)
I have two HDD (1tb and 2tb, total 3tb but both are pretty old and slow)
Ok here's the thing.

1. On bios my ram appears to be 8gb (I already checked my dimms by switching the ram sticks with every possible combination and both sticks work fine)
2. The hardware seems to reserve the exact same amount of ram (4.1Gb is reserved and 3.9Gb is available for me to use through tasks) on windows 10 and after formating my pc and clean installing ubuntu. So two different OS same issue.

What can be the core of this issue just out of curiosity? I'm really beginning to become obsessed with this issue!
My next move was to buy an RX 570 (8gb) gpu and an 240gb SSD (still waiting both to arrive) with the hope to slightly improve and maybe resolve this issue.
I'd like to ask if the outdated hardware I have like HDD or the gpu can cause 4gb of ram to be reserved? Is something like this possible?
Sorry for asking silly things like this but even if I totally buy a new PC I really would like to know what is causing it!

Thanks in advance

 

[GearDoGi]

For anyone seeing this I think I got an expanation but can anyone with more knowledge confirm it?
So my MoBo has 4 available memory slots and I use 2 of them. The corsair suggests its should be 1600mhz but the pair I use is 1333mhz could this cause the issue I described above?

 

[Calvin7]

You likely have two 1x4GB memory kits.
XMS3 — 4GB (1x4GB) DDR3 1333MHz C9 Memory Kit - Corsair

You haven't identified your memory with a part number.

 

[GearDoGi]

You likely have two 1x4GB memory kits.
XMS3 — 4GB (1x4GB) DDR3 1333MHz C9 Memory Kit - Corsair

You haven't identified your memory with a part number.

In that scenario is it possible to use two of the same kind as dual channel?
Is there any positioning on the mobo slots to make both single module ram work together?