4 sticks of RAM gives a total of 12GB?
Hmm... Well usually RAM comes in kits of either two or four sticks. These kits are matched in the factory to ensure compatibility so when one becomes faulty it's usually best to replace both sticks (or the four depending on set up).
Also ram nowadays (for pc) comes in either 8GB or 16GB sticks so what your saying does sound a bit unusual.
What I’m trying to say is that only one of the two sticks of RAM were faulty so the working one was left in. Then two more new sticks were put in so there’s one old stick and two new sticks in the PC so three in total. Each one of the three is 4GB.