If I am correct, you want to know the power transmission at 4um away from the mode source for an infinitely long waveguide. If so, it is absolutely no need to simulate it with FDTD. Instead, you can get mode property from FDE and get the loss constant. https://optics.ansys.com/hc/en-us/articles/360034917493
I am not sure what kind of coupling you are refering to. For such waveguide, there is no any change of geometry or surrounding index, is there any other mechanism for the coupling? Maybe you did not give all the details? Please note, when you calculate the mode source, it uses all the materials composing this waveguide. Without geometry or index change, for such a single waveguide, I do not see any physics that creates some kind of coupling. Please think it over and elabote more.
You may refer to the mode analysis here: