12 ohms load?

[milesmiles]

I have a 2 channel, 100 watts into 8 ohms per channel, Sony receiver in my house. Currently I have two 4 ohms speakers wired into the receiver.

I'm going to be getting a pair of Sony bookshelf speakers for Christmas. They are 8 ohms. I am trying to figure out a way to wire both pairs of speakers into the same receiver.

Although the reciever is 2 channel, it has 4 outputs. Engaging all four outputs at once is, according to the manual, the same as wiring in parallel.

According to my calculations, wiring the two loads, 4 ohms and 8 ohms, in parallel, would present 2.667 ohms, which is much too low for the receiver.

Wiring in series would produce a load of 12 ohms. I am not sure if this will do the bookshelf speakers, which although have no RMS rating, have a maximum input of 120 watts, which is above what the receiver produces at 8 ohms already.

Do you think I would get decently loud and quality sound with wiring in 12 ohms? Or should I go ahead and buy another amplifier for the other two speakers? Thanks for any input.