Why would I need to change the layout?
Every time I upgrade the desktop for a new Ubuntu version it defaults it to a 2×2 layout.
I don’t have a monitor on top of another so why would I use vertical workspaces? It maybe has advantages for some else workflow. Not mine. I’m used to horizontal multiple screens.
How can I define a new workspace layout?
You can change several parameters with the Compiz Config Settings Manager. I found that some friends didn’t have it installed. So, just install it first.
sudo apt-get install compizconfig-settings-manager
Call it pressing your keyboard Super key and typing compiz. When it appears click on it.
CompizConfig Settings Manager window shows up. Now just select General / General Options / Desktop Size.
To change your workspace layout you just have to call CompizConfig Settings Manager and go to General Options / Desktop Size bar. Change the Horizontal and Vertical Virtual Size to suit your needs.