If you first buffer each transducer so you have an output going to your metering circuits. You could normalize your gains at this point. Then four rectifier circuits, mean RMS or peak according to how you want to compare the signals. One way forward then could be to apply each of the signals to say the positive input of a comparator chip (four chips) and a slow frequency sawtooth to all the negative inputs of all the chips. Then ramping the sawtooth from say -10V to 0v would provide an output on the chip with the highest input. This could then stop the sawtooth and energise a switch that allows the highest signal through.
Problems are the signals time synchronous?, slowly moving DC?, How often do you want the circuit to check to see if its lselected the highest value?
Frank