I posed this question directly to Matlab support and they came back with the following answer, which, paraphrased, says, "No, you can not specify parameters as constants with built-in model options."
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I understand that you would like to use "fit" function with custom "fourier4" model. You can supply a custom "fit" function to get the required type of model.
To fit a custom model, you can use a MATLAB expression, a cell array of linear model terms, an anonymous function or create a fittype using "fittype" function and use this fittype as a parameter to "fit" function.
Please take a look at the following example to fit a custom model using an anonymous function: http://www.mathworks.com/help/curvefit/fit.html#bto2vuv-11
You can also take a look at the following examples on creation of custom linear and non-linear models: http://www.mathworks.com/help/curvefit/fittype.html