Allan, as Blutarsky mentioned, the Horizon is not a 4 ohm speaker. For speakers, impedance will vary with frequency. 8 ohms speakers will typically dip down into the 4 ohm range for some frequencies (minimum impedance) and peak as high as 40 ohms at other frequencies. For 4 ohm loudspeakers, they will typically dip below 3 ohms, sometimes even below 2 ohms for certain frequencies. Minimum impedance is not what determines the overall impedance rating for a speaker.
The Horizon presents an easy "8 ohm average" load to an amp or receiver. It also has a very mild phase angle - the critical factor in determining how difficult a loudspeaker is to drive. No concerns running the Horizon off your Yamaha, or any other receiver for that matter