I built my computer around November of 2016. I only got 8GB of ram because I thought that would be fine for some time. I now want to upgrade my computer's memory, but the RAM I bought may not sell anymore. The ram is 2 x 4GB of Avexir Core Series DDR4 RAM (red). Is it okay to mix other sticks in, or do I have to buy four completely new sticks? Anything helps!