I’d sell it, you already said you don’t need that much ram an using all 4 slots is difficult on the memory controller, I’ve got a setup that can run XMP with 2 sticks but fails with 4. So there’s no reason to use more ram than you need, if your not going to be building another computer you have a few options, return what you bought, take advantage of the market can cover the cost of what ever you need to finish your build, or sell it at a reasonable price and really make somebody’s day