It's a bit complicated. A speaker that is "rated "at 8 ohms actually isn't always 8 ohms. At some frequencies some speakers can drop down to just a few ohms, like 2 or 3 (or even less in some instances), and at other freq's it can be much higher then 8 ohms. When a speaker drops to a lower resistance, it demands more current from the amplifier. If an amp can't provide that current, it is often described as not being able to drive that speaker.
Ideally, an amp should produce twice as much power when driving 4 ohm loads as it does at 8 ohms. If you look at the specs for most amps, you will see not that many amps can do that. Thats as much a problem with the speakers design as it is with the amps design. Some speakers designs are known for being demanding loads for any amp.
I looked for an 4 ohm power output spec for this amp, and didn't find one (thats not unusual), but if I had to guess, I would say that it can drive 4 ohm/ 86 db speakers for typical listening. But that also depends on how loud you like to listen. Before buying one I would wait for further info and reviews, or e-mail Nuforce with your question, if this is important to you. Nuforce's other amps do have a good reputation with reviewers, however.
More info-
Loudspeaker Impedance
more detailed info on speaker impedance and power-
Home Toys Article - Speaker Impedance, Your Amplifier And You.