Some Hartke speakers apparently have a dual voice coil design which allows impedance switching. Here's an extract from a Hartke manual:
"The HyDrive HD112 has an innovative dual voice coil design that allows the cabinet to operate at either 4 or 8 ohms impedance. The cabinet has a impedance selector switch on the rear jack plate. If you are using a single speaker cabinet, and the amplifier has a minimum impedance of 4 ohms, set the HyDrive HD112 impedance selector switch to 4-ohms to get the maximum power from the amp. If you are using two HyDrive HD112 together in parallel, for a total impedance of 4 ohms, or adding a HyDrive HD112 to your current rig, set the impedance selector switch to 8 ohms."