From the single camera view in portrait (with your head turned sideways), tap the closed box icon in the upper right to rotate to landscape. Then tap the back button, upper left to return to four cam view in landscape. To undo this, go back to single camera view, and tap the open box icon top right. (You need to tap the screen each time to bring up the controls.) This allows you to get in and out of landscape without picking the tablet out of the stand.
That said, we really need full landscape support for tablets. You can vote for that here (be sure to click the VOTE button at the top left):