Is over 4gb useless? Well, I have 24gb and running Windows 10 and Forza Horizon 3, I use about 10gb of it.
As others mentioned, 32-bit Windows = Max 4gb, 64-bit Windows = Max 18.4 exabytes(18 million terabytes), but as you can imagine, there are other limitations like how much a motherboard and CPU can support and the physical size of having such a massive amount of RAM.
Right now, over 16gb is pointless for gaming as games don't use that much and the rest will go unused. I went with 24gb because I tend to keep my systems for a long time and I decided to max out my motherboard. Above 16gb is a good idea if running productivity software that can use it.