Shooter, welcome. The response you got from the Onkyo rep is total nonsense. He apparently was trying to interpret what the manual suggested, but he had no understanding of the technology involved. If there was a problem with using speakers with different impedance ratings simultaneously(there isn't)using 4 ohm with 8 ohm would be less of a problem than using 4 ohm with 6 ohm. The point is that the overall net impedance, taking all the speakers into account, determines the amount of current used at a given volume level.
There are no problems with lesser units than your 5009, and you certainly should experience none. In any case, keep the impedance setting at the default higher number, since setting the lower one simply restricts the amount of voltage available from the receiver's power supply section. This results in limiting current and power available(following Ohm's Law)and reduces the chance of overheating, but at the expense of limiting maximum performance capability(tests show that the lower setting cuts maximum power output roughly in half).
