https://www.gamespot.com/articles/new-nintendo-switch-update-out-now-heres-what-it-d/1100-6454427/

Nintendo has rolled out another firmware update for Switch. Version 4.0.1 arrives just a couple of days after the system's big 4.0.0 update, though it isn't nearly as exciting.

Unlike the previous update, which added several new features to the system, update 4.0.1 is another minor one that primarily for "system stability improvements to enhance the user's experience." According to the patch notes, which you can see below (as taken from Nintendo's support site), the update also "changed the HDMI connection process specification to match that of version 3.0.2 to address video and sound output issues with some TV models."