What does Task Manager show when your computer is idle (nothing else open)? I had no issues with only 3 GB of usable RAM on a Windows 10 system, with around 2 GB free to use, and the only time I've come to maxing that out was when editing a huge image in GIMP (even the 64 bit version of Paint claims there aren't enough resources when opening very large images).
When i close everything, disk and processor are both under 5% most of the time, RAM is just under 40%- windows antimalware service is running always when i open task manager. When idle most RAM uses also the task manager, windows explorer and registrers.
Upon starting just google my proccesor spikes up to 98%, hard drive to 40% and ram to 98% as well
Also interesting thing, for the first time i noticed in task manager it says i have 4 Gb of RAM, but i have 2 ram cards, both 4Gb and i was told it should add up, so i should have 8 and not 4Gb ram