4k vs. 1080p on a 1080p monitor
No, there is no difference to the human eye (or exceptionally minimal).  Your monitor can't display higher quality than it is capable of displaying.  The only advantage you would have is if you were to zoom in on part of the image, you would have more detail when you zoomed in.

The exception to this is if the 4k stream uses enough extra bandwidth to reduce the number of artifacts present in the stream, but for a well configured stream this shouldn't be a significant factor.  For Youtube settings, that may or may not be noticeable depending on your sensitivity to artifacts and the type of content you are viewing.