sony amp

It refers to the loads that you are putting on the amplifier.  4/2 and 2/1 refer to the impedance of the loads.  You have two loads attached:  two 4 ohm subs on one side and 4 speakers on the other.  As for the speakers, it can be assumed that you are putting a 2 ohm load each onto two channels.  You probably have 4 ohm speakers and have paralleled the pairs, right and left.  That's a "2" and can go with either of the choices (4/2 and 2/1) as they both include a "2"...which means that if those speakers were the only load on the amplifier you could use the switch in either position.

As for the subs, how do you have them wired?  Are they paralleled to 2 ohms or are they series to 8 ohms?  Answer that question and you will know which of the switch positions you should choose.  If they are paralleled the switch should be set at 2/1...if they are series the switch will be set at 4/2. ...(the load of the subs represents the "4" or the "1" in the problem   Parallel wiring to 2 ohms and then bridging onto the amp puts a 1 ohm load onto each channel - therefore the "1".  Series wiring to 8 ohms and then bridging onto the amp puts a 4 ohm load onto each channel - thus the "4").

At least, that's my best guess.