The Allison Four woofer is actually 4 ohms impedance. In the model Four, there is a 2.2 ohm resistor and an inductor in series with the woofer which raises the impedance to 8 ohms.
Here are the specs for the Allison woofers. The model Four woofer appears to be unique but the LC110 or LC120 woofer may be a fairly good replacement. If one could be obtained at a reasonable price, it may be worth a try.
Allison Driver Specifications