Why does Windows only show about 3.5 GB of my 4 GB of RAM?

You can't: See Dude, Where's My 4 Gigabytes of RAM? if you want to fit memory and devices into a 32-bit address range: not all of the available 4GB of address space can be given over to memory. So what actually happens if you go out and buy 4GB of memory for your PC? There's a hole in your memory map for the IO. (Now it's only 25% of the total address space, but it's still a big hole.) So the bottom 3GB of your memory will be available, but there's an issue with that last 1GB. The only practica…