The simple answer? It would be safer to set it to 4 ohms on your receiver.
With impedance, you want to try and avoid connecting speakers that have a lower impedance than the AV receiver is designed to handle.
This is because a lower impedance will require more current from the receivers power supply than it is designed to output.
Therefore if you set the AV receiver to 4 ohms, then your 6 ohm speakers will have a higher impedance - which is safer than having speakers with a lower impedance connected to it.
However, this is all theory. In the real world you would probably be fine have the receiver set a 8 ohms - as long as you didn't turn up the receiver to maximum volume for long periods.
But as you have the choice of 4 or 8 ohms, it would be better to have it at 4 ohms to be on the safe side.