A windows block is a window that can contain several views.
Select the image you want to visualize. Open a 4 views block using the Windows => Open a 4 views block menu.
You can add a new window to the block by dragging the window item from Anatomist's list of windows and dropping it in the block.
To remove a window from the block, use the window menu Window => Detach view.
It is also possible to reorganize the block by changing the views layout, using the options in the block window menu.