Adrock,

It will cause NO problems whatsoever if you drive 4-, 6-, and 8-ohm speakers simultaneously by the same receiver (assuming that your Yamaha will tolerate 4-ohm loads in the first place, which I believe it will). Also, you will NOT damage a speaker by driving it by an amp that is "too powerful" for the specs of the speaker -- in fact, the vast majority of speaker damage is caused by an amp that is not powerful enough and thus forced into clipping. An overt amp clipping often damages the tweeters.

Finally, if your receiver has a switch for speaker ohmage, simply leave it at the "8-16 ohms" or "high" position, unless the receiver ever shuts itself down in the middle of a playback. All this switch does is to lower the amp's power-supply rail voltage a bit when flipped to the "low" position, and therefore decrease the max available power from the amp -- you don't want to do it unless you need to (i.e., thermal or over-current shutdowns).