On each time step of the simulation, the entirety of the code in your MATLAB Function Block will execute to produce the output y. This is why its value is always 4.
If you need access to the simulation time you can use a Clock Block:
as the input to the MATLAB Function Block. Then, on each time step, the input u will be the value of the current simulation time.
As a suggestion, making if an extrinsic will likely have unexpected consequences so removing that may be best. Similarly, min is supported for code generation so there should be little need to make it an extrinsic function.
