I studied with this guy about ten years ago in MaxMSP. He’s got some great (free) software on his page.
The OP-4 is an instrument consisting of four Teenage Engineering OP-1 Synthesizers. They are being controlled externally from modular software in Pure Data.
I think we’ve all got four OP1s lying around havent we?
I know that this is not free but you need to try new Reaktor 6 with blocks. Great sound quality for digital stuff!