If you've got your rear surrounds hooked up and no sound is coming out, it has nothing to do with the speakers' ohm rating. Something else is wrong. Recheck your connections. Did you try it with a test tone to your surrounds? If nothing else checks out, you may have a faulty receiver.
As crashguy said I woudn't worry too much about the 4 ohm rating. There's really no such thing as a constant 8 ohm or 4 ohm speaker anyway. The resistance level varies as a function of frequency. For example, your "4 ohm" speaker may at certain frequencies be presenting a 16 ohm resistance and at other frequencies 2 or 3 ohms. The 56Txi is generally considered to have a strong amp section for a receiver. If the sound quality is OK and your receiver isn't going into protect mode, don't worry about it especially since your "4 ohm" speakers are surrounds. Compared to your front mains and center, very little content goes to your surrounds.
