Using fitnlm with multiple predictor variables

I am trying to perform non linear regression using 4 independent variables and a 2-term exponential model. I am using the following section of code:
X = [x1 x2 x3 x4];
b = [1 1 1 1];
x = ones(size(X));
modelfun = @(b,x) b(1,:).*exp(b(2,:).*x) + b(3,:).*exp(b(4,:).*x);
beta0 = ones(4,1);
However, I keep getting an error explaining that modelfun returns a vector of 4 columns where it should be 1 column.
I have tried as many variations of modelfun as a I can think of, can someone help?

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