not sure, but maybe a workaround (for testing at least) could be found using filtergraph~ or techno~, they each have some of that X Y capability for moving and stretching arbitrary points/shapes. For a true implementation you’d probably have to go with js.
You also don’t *just* need X and Y, you have a total X size with all 4 sliders, and if you change one, the others (maybe just those above it) will stretch to fit? Or can the bands be separated and/or overlapped? If so, your job is a bit easier as they won’t be tied together.
You might need to have multiple objects layered atop one another, which means you’ll have to select which one you’re manipulating. That’s an extra step but it could allow you to make each slider more easily, possibly a workaround with multislider or pictctrl.