I have an old m4 brake which has I.S. mouonts rather than the post mounted calipers normally seen now.

With this brake am i limited to the size of rotor, it seems to only fit a large rotor (203)

Are adapters available to use smaller rotors and how would they work?

bit confused as i know how to go up a size with post mount brakes but cant figure out how to come down a size with the hopes when it has a fixed size I.S. mount.

The old Hopes (C2s etc) you could change the half-caliper with the mounting lugs. But I don’t think you can on the M4s, even the old ones.
Due to the way they work, you can get adaptors to go up a size, but not to go down. One of the reasons why IS isn’t a particularly great design, I’m afraid you’re stuck with big rotors.

BUT, there’s demand for M4s, you’d have no problem selling it, or swapping it for a different calliper/brake setup.