Why don't use simply a 4 channels EQ ?
MCFX's is quite good and free :
http://www.matthiaskronlachner.com/?p=1910
and I think that the MEqualizer supports up to 8 channels
https://www.meldaproduction.com/MFreeFXBundle
You can find also a 4 channels version of the ReaEQ on Landolleet
https://www.landoleet.org/