You get your introduction mini room, and afterwards you are droped on 4 rooms and you must make a laser travel through all four rooms to open your exit.
Download from Thinking With Portals here.
Put the .bsp on your maps folder at steam/steamapps/common/portal 2/portal2/maps. Then to launch the map open your console and type in "map mp_motanum_main_four". It should drop as you type it.