A receiver doesn't have an impedance it usually has a range of impedance that it will tolerate. Going higher than 8 ohms is never an issue. Going lower than four or six ohms can be.
If you wire them all in series you will reduce the power of the receiver , degrade the bass performance and only use two channels of the 5 or 7 you might have.
If your receive will tolerate a 4 ohm load and it has 7 channels them just connect them individually and use 6 of the 7 channels to drive them. If it won't take a 4 ohm load then
you should be able to connect two pair to the front in series and two pair to the rear in series.
Set the surround mode to all channel stereo (or a similar mode).