Dowscaling to 480p? Not necessary with NVENC, since it offloads the encoding to the GPU, which should do at least 1280x720 just fine, atleast my GTX 770 does.
Also 4000 kbps bitrate is too low for NVENC, it's quality is worse then x264 at a given bitrate, although it might not be so noticeable at 480p.
Also the local recording guide is THE way to get the best quality videos, if you have the cpu power to use x264 and play a game at the same time. For NVENC you can achieve close to that quality with a high bitrate.