The plant model has 4 outputs and 1 input. If you look at
you'll see that the second (nY) and third inputs (nU) are the number of measurements (y) from and control inputs (u) to the plant. But the plant has additional inputs (w) and outputs (z) that define the the closed loop transfer function for optimization. So the plant has to have at least 2 inputs and 5 outputs altogether, which is what the error message is saying.
Having said all that, are you sure musyn is the correct function to use? I'm asking because
a) the comment says "Robust Analysis" and musyn is used for robust design, and
b) the outputs from the call to musyn don't seem to match any of the signatures on doc page.