4 ohm speakers are generally a more difficult load to drive, and the sensitivity is on the low side. That means that (1) the amp sections of many receivers may have difficulty driving the speakers well, and that (2) the speakers are likely to be most effective if given a good bit of power. The Model 770 would have no trouble with a 4 ohm load, and at that load it is rated at 300W per channel, which should be plenty of power for these speakers.