You can generally fudge the impedance a little bit, running 4Ω loads on 8Ω taps and vice versa. Just don't go too far with it. Running 2Ω on an 8Ω tap is a bit much, for example.Ohm's law says each time you connect spekaers in parallel, the impedance is halved. Each time you connect in series, it is doubled. You can hook a pair in series, and a second pair in series. Connect to each end and impedance is doubled, as per Ohm's law. Now if you connect the two pairs in parallel, then cut that back in half to the original value.