That's hard to say without a lot more information. As to what the 4-8 ohms means, I frankly don't know what MS interprets that to be. For example, the impedance plot of their Performance 6 looks like this.
The speaker's impedance (the resistance as a function of frequency) varies from a low of about 3.5 to a high of about 10 ohms. Now, if you're going to use a sub, set the speaker's to small with a crossover of about 80 hz, sit maybe no more than 3 meters away, and not routinely listen above reference levels, a receiver that's only spec'd to 8 ohms just might cut it. The thing is, your listening preferences are unknown as is your room and for that matter the receiver you plan to use. Power ratings on receivers tend to be a bit optimistic and I don't know what the SR606 is capable of doing especially at 4 ohms. What are you using at the present time for a receiver?