Private Work A Work B Work C
Ideally it would seem like a different user. (Filesystem, cmd-tab). But easily accessible like the three finger swipe. Fast user switching doesn't cut it.
I even tried logging in on my local machine using VNC or remove desktop.. Is having 4 VMs the only way?
KDE Plasma has what they call "Activities". In its basic idea, it's kind of like fancy desktop workspaces. So, it does separate your windows into workspace-like groups, but you can also set what files and widgets are displayed on the desktop on a per-Activity basis.
So, you don't have a different filesystem, but you can have a folder or multiple folders displayed on each Activity's desktop. Also, shortcuts to different applications, including for example Firefox profiles, as well as different sets of widgets, for example I like to use the little post-it note widgets to write things down on.
And yeah, with that you can then just switch between Activities via a simple keyboard shortcut or by clicking an Activity-selector on your panel (if you've put one there).
Long time ago (win2000, before XP)I tried to do it on windows with the "runas" command, but things like IE would keep popping up with the wrong user (they managed to communicate with the desktop and have that open the new window) so it wasn't a good solution. Perhaps it is better now.