You can add a resistor by connecting it in series with the speakers, but it would have to be a high-power resistor. As for the volume, if you have 4R instead of 8R, you can't turn up the volume too much, if you can already hear the distortion, you should definitely turn it down so as not to break the power amplifier. i.e. theoretically, you can turn up the volume until distortion starts to appear, no further, but I do not recommend connecting 4R speakers to the 8R amp at all
