4 ohms , 6 ohms, or 8 ohms???

While Erik's point regarding being very careful in buying any tube amp for this speaker pair is certainly good advice and true, it should be noted that there are also tube amps that could likely annihilate and disintegrate them as well. Like this one http://www.atma-sphere.com/Products/#MA-3

That said, I'm in the SS camp.

In answer to your question about whether or not it would be preferable to hook up to an amp via 4, 6 or 8 ohm, my opinion would be to go with "8" to essentially lower (with respect to 6 or 4) the current output of the amp and in so doing make its life a little easier per se and lower your electric bill. However, I'm not certain if there is an audible advantage in using a lower ohmic value connection so I'll await the input of other more knowledgeable folks than I on this forum. I don't think there is an audible difference.

No worries, unless you live and breathe this stuff those graphs don't make a lot of sense. :-)

And while the graph does have an impedance below 4 ohms for most of the bass up until around 700 Hz, the operative word is "most". There is also a small range (say 70-90 HZ) where the impedance is up above 10 ohms. So the question raised as to which ohmic value to hookup to on an amp is certainly worthwhile trying to understand.  

Almost all speakers have varying impedances, and there is no 1 right way to judge what a speaker's "nominal" impedance is.  Some by average, some by minimum. There's a great deal of spec doctoring or wishful thinking involved.

You want to pick a tap that is closest to the measured values, so for this speaker 4 is it. However, it would be worth listening to other taps.

The issue isn't the impedance peak, but the minimums. I wouldn't call this a 4 Ohm speaker, I'd call it a 3 Ohm speaker. It's really abnormally, and broadly low in impedance to call it anything more.  That most of the bass is below this range means the amp is going to be working hardest where the music amplitude will be greatest.

Interestingly, Stereophile reviewed these speakers with a McIntosh with 2 Ohm taps, that seems much more appropriate than 4 Ohms. Also interesting was the change going to the Bel Canto's which don't have very good low impedance drive.

You want something pretty stiff. Less expensive but very good alternatives in the SS land would be the Parasound A21 stereo amps. That or better is where I would go.

Honestly I don't like the impedance curve at all, but if you like the sound you are going to have to carefully match them to the right amplifier to get the most out of them.

I believe, Ralph of Atma-Sphere, recommends using a Zero device for low impedance speakers.Here is a link
 http://www.atma-sphere.com/Support#FAQ
On the right hand side, he discusses what you are asking about. The Zero link is highlighted in blue.