You may need a new CPU and motherboard. Older systems just won't do more than 4G of RAM.
(By the time you replace the CPU and motherboard, you might as well buy a new computer.)
(Now is a good time to buy an "obsolete" computer running Windows8 at some retailers.
I picked up a "sale" W8 laptop, applied a "$75 off any windows 8 system" coupon, and ended up paying about $175 (At Staples. They now have a $100 off W8 coupon, but it's less clear whether it will apply to sale systems. I'm not entirely sure I SHOULD have been allowed to used the coupon, but I'm not complaining...) (This does probably require a retailer that bailing out on anything pre-W10.)
Look for my thread under "cr0sh" - all-in-all, I managed to "drop the price" (with all the discounts and cash-backs I got) from $240.79 to $113.56 - that's saving over 50%! For a quad-core I5 Haswell, that's a pretty smokin' deal.
Ultimately, it's going to be for a GTA5 gaming box...