In high gain circuits noise reduction is an important consideration. Paralling four amplifiers will reduce the total noise of the circuit by a factor of two. The output noise of each amplifier, enOUTx, is attenuated by four and summed at the output terminal. Since noise from uncorrelated sources is summed in RMS fashion the total ouptu noise is: enOUTtotal = sqrt((enOUT1/4)^2 + (enOUT2/4)^2 + (enOUT3/4)^2 + (enOUT4/4)^2) which is sqrt(4 * (enOUTx/4)^2) = 2*enOUTx/4 or enOUTx/2. One half the noise of a single amplifier.
