why not use 4 synth voice modules like these http://www.modulargrid.net/e/analogue-solutions-stationx http://www.modulargrid.net/e/doepfer-a-111-5 http://www.modulargrid.net/e/acidlab-m303 http://www.modulargrid.net/e/pittsburgh-modular-synthesizer-box http://www.modulargrid.net/e/atomosynth-koe-v2-0 and a mixer like this http://www.modulargrid.net/e/circuit-abbey-unify with the relevant midi interface

you can pick and choose what you like it is modular after all