I don't know about 4 ohms. You might even go the other direction, 16 ohms? On the back of my W70s was scrawled 7-16 ohms. Not factory writing, but the earlier wharfedale's were 16 ohm, and quite a few of their early drivers are 16 ohm. I have a pair of Ampex el 34 monoblocks that are 16 ohm output, and I need to get working on my w70's to try those out on.