If you only have 4 speakers, you should plug two of them to the Main front channels and two of them to the surround channels of your receiver. Then turn off the center and surround back channels. Calibrate the 4 speakers using the internal test tones and an SPL meter. You can run 4CH stereo when listening to CD's or MP3s for more impact. Peruse the Audioholics Editorial site for more setup tips.
Down the road you may want to upgrade your speakers and add a powered sub for more bass impact.