For (4), since the receiving screen is on the focal plane of the lens, the parallel rays with the diffraction angle θ with the optical axis converge to one point of the receiving screen (θ=0 degree for the rays forming the main maximum, that is, the rays parallel to the optical axis). When the single slit moves up in parallel, the light whose original θ is 0 degrees also moves up in parallel, but it is still parallel to the optical axis. According to the lens theory, parallel light converges to a point on the focal plane, so the convergence point of these lights is still the original position.