1) Pagefile has nothing to do with how much system memory is accessible.
2) Either it's a SOFTWARE issue, or else you have 32-bit Windows 10.
With 32-bit you have a 4GB memory map. The amount you describe only makes sense if the VRAM on the graphics card (or onboard usage) is about 512MB. If that's the case then there's not much you can do.
3) If you have 64-bit Windows then type in "msconfig" launch that and go to BOOT-> Advanced Options.
Try changing the "Maximum memory" there to I guess 4096MB.