I found this script to be UBERuseful, which will switch between the viewport under the current mouse cursor location and the quad view.

script:if ( moi.ui.mainWindow.viewpanel.mode != 'split' ) { moi.ui.mainWindow.viewpanel.mode = 'split' } else { var viewport = moi.ui.getViewportUnderMouse(); if ( viewport ) viewport.viewpanel.mode = viewport.name; }

..just put it on the SPACE key or (almost) any other key combination like so:

http://moi3d.com/2.0/docs/moi_command_reference11.htm#shortcutkeys

Cheers,
Marco