Normally receivers that have a speaker impedance switch employ it as a current limiter so that the receiver will not overheat if the impedance gets too low. However, dynamic range can be reduced with this current limiting in effect (6 Ohm position).
I usually recommend (and especially since you are using the receiver only to drive the 4 Ohm center and a pair of 8 Ohm surrounds) that the switch be left on the 8 Ohm setting, and that you monitor the heat produced by the receiver.
If it doesn't feel like it's running too hot, leave it at the 8 Ohm position, but if it seems like it is running too hot, switch it to the 6 Ohm position to reduce the possibility of overheating.
Steve