Probably the only safe way to do it is run them in series. Specs say those are 8 Ohm speakers, so running them in series would double the Ohms and halve the power. Running them parallel would halve the Ohms and double the power, but I doubt the X4000 is capable of 4-Ohm loads for very long.
I'm confident the x4000 can do 16-Ohms and since we are only talking 4 speakers, maybe get 50 watts RMS per speaker. I wouldn't run anything on Zone 1 while doing this though.
Wiring in series. (+) from AVR to (+) on Speaker #1, (-) from AVR to (-) on Speaker #2, and connect speaker wire from (-) Speaker #1 to (+) Speaker #2.
Another thing to contemplate is any automatic room correction ( like Audyssey ) you'll be using. It might give you some strange results IF the two speakers are far apart but shouldn't be a problem if stacking speakers.