From Ultimate AV:
In reply to:
The Yamaha Rx-Z1's stereo analog frequency response, left channel measured from the CD input to the speaker output, was -0.03dB at 20Hz, -0.09dB at 20kHz, and -0.41dB at 50kHz. The Dolby Digital response (optical input to speaker output) measured -0.05dB at 20Hz and -0.18dB at 20kHz, left channel; the center channel was virtually the same. With the left surround channel set to Small, the response was -3dB at 91Hz and -6dB at 69Hz. The A-weighted signal/noise ratio measured 100dB. THD+noise in stereo at 1W into 8ohms measured 0.007% at 20Hz, 0.009% at 1kHz, and 0.006% at 20kHz. At 2W into 4ohms, THD+ noise measured 0.005% across the board. With five channels driven, the Rx-Z1's power output at clipping (1% THD+noise) measured 137 watts per channel at 1kHz and 132Wpc at 20Hz, left channel measured to the nearest watt. Driving all channels into 4ohms, the Yamaha clipped at 188Wpc at 1kHz and at 169Wpc at 20Hz. With two channels driven at 1kHz, the Yamaha clipped at 153Wpc into 8ohms and at 258Wpc into 4ohms.
They got it to clip which is fine with what they do, but there is no mention of the receiver shutting down as a result of the 4ohm load. Like I said, you will be fine.