Question : Problem: How can I be logged in with multiple Windows users simultaneously?

I would like to be logged in with 4 windows users simultaneously and have access to all their desktops and files at once.. basically on a 2x2 grid on one screen.

I do not simply want software that will allow me to have multiple virtual desktops.  I need specifically to have each desktop belong to a different user, and act appropriately.

If it's not possible to have them all open at once, is there a method for super fast transitioning between users, say as fast as you can transition between windows within the same user?

I will not be doing this via Remote Desktop or remotely at all.  I will be physically at the machine.

Answer : Problem: How can I be logged in with multiple Windows users simultaneously?

Set up a Windows Terminal Server with 5 CALs.
From your workstation, run RDP in a window (not full-screen). Login as User1.
Repeat for User2, User3, User4.

This gives you 4 sessions on one computer.  If you have the horsepower, you can install Server 2003 or 2008 as a Virtual Machine on your existing workstation.

Be advised, you'll have to buy additional application licensing for each CAL.  So, for MS-Office, you'd need 5 licenses to match your 5 CALs.