I have a Yamaha RX-V683BL and I would like to plug four 8ohm speakers into the zone 2, left and right, that's 2 speakers to the left plugs and 2 speakers to the right plugs. This will be for the outside patio. Is this safe for the receiver ? Do I need a speaker selector switch ? Inside the house I will be using the rest of the plugs for the 7.1 surround.
If you use the receiver in 7.1 surround your would lose 2 channels in the surround system. The receiver should switch to 5.1 when you turn on powered zone 2 but check that it goes back to 7.1 when you turn zone 2 off.
The receiver does give a dynamic power rating into 2 ohms so it should run the 4 ohm load. Whether it will play them loud enough or shut down while trying to do so is another question. There's really no way to know how much power the receiver can deliver.
Worse case you would get an amp to connect to the line level zone two output of the receiver.