Rated 5 out of 5 stars

It works great, but I have one question. Why does it not download MP4s in 480p?

This review is for a previous version of the add-on (1.3.2). 

Because YouTube doesn't offer it. A few years ago, all the videos were only available in the FLV format (mostly 240p, then 360p and 480p). When YouTube started to convert its videos to MP4, it still kept the FLV videos, which are compatible with older Flash players.