In sector mode, hover the mouse near the top edge of S4. Find the spot where the sector outline turns white instead of orange. Drawing counterclockwise, create a new sector inside it, the same height but only 64 across. Press L, left click the left and right sides of this new S6; both should turn red. Right click either, hit "select action," expand "Door" and select "D1 Door Open (Red) Stay." Go to "sidedefs," click on the top left pane and select BIGDOOR2.