Can Snap-Windows be set up for 4 windows instead of just 2?

I think you can edit the EDGE settings in GRID using the CompizConfig Setting Manager (to install this run sudo apt-get install compizconfig-settings-manager in a terminal).

If you set the corner setting (i.e setting Upper Left Corner to Top Left Corner) then when you drag the windows to the corners, it will "SNAP"

Ok found the option using Compiz (We need CompizConfig Settings Manager installed). Open CCSM and go to the Window Management

The plugin we will use is the Grid plugin. Open it and select the Edges tab.

Now change each behavior until they look like the image above so that:

Upper Left Corner - Top Left Corner

Upper Right Corner - Top Right Corner

Bottom Left Corner - Bottom Left Corner

Bottom Right Corner - Bottom Right Corner

Now logout of your system and log back in. This is to avoid any weird graphics effects.

Now what you have is a system where, if you drag a window to one of the corners, it will get assigned to that corner and not the half of the screen. So dragging a windows to the bottom-left will assign the window to the bottom-left grid. Dragging it to the top-right corner, will assign the window to the top-right grid and the left and right screens (Left and Right halves of the screen) will still work just by dragging the window to either the Left or Right of the screen.

The final output should be something like this: