I am sorry, I do not understand your first constraint. You say "c(1-4)>=1" is the constraint, but this is neither mathematical nor in MATLAB notation. Perhaps you mean the sum of the values is at least 1: sum(c) >= 1. But maybe this is a complete misprint, and you really mean that all the c(i) are at least 0. You represent this as a bound constraint:
lb = zeros(4,1);
To represent the equality constraint, you did the right thing:
Aeq = [0,0,1,1];
beq = 1;
You should change c0 to be feasible:
c0 = [3,30,.5,.5];
The error you have is because your objective function does not return a scalar value. Use the following:
fun = @(c) sum(yhat(c,t) - y).^2;
Now you are ready to proceed:
c = fmincon(fun,c0,[],[],Aeq,beq,lb)
plot(t,yhat(c,t),t,y)
You will see that the fit is not very good. This is primarily because your data has y(1) = .82, whereas your restriction c(3) + c(4) = 1 imply that y(1) should be 1.
Good luck,
Alan Weiss
MATLAB mathematical toolbox documentation
