Your situation is familiar, since many people do not have large rooms for their home theaters.  My advice is to place the rear ceiling speakers very near the rear wall.  This will cause much of the speaker’s sound to reflect off of the rear wall improving the sense that the sound is coming from behind them.  The front speakers should be placed normally in the ceiling according to Dolby specifications which allow an elevation of 30° to 55° above the horizontal plane of the listeners ears.  I recommend using the more distant ceiling position of these speakers (closer to 30°) to minimize localization.”