Re: What did the new WiiU update do?

https://www.nintendo.com/consumer/systems/wiiu/en_na/system_system_update.jsp?menu=general_info&submenu=wup-online-gi-update

Judging on how long it took to download/install, it was a very small update and just for some "system stability" improvements for the Wii U console.

Version 4.0.3 U, available via an Internet connection, includes the following improvements:

• Further improvements to overall system stability and other minor adjustments have been made to enhance the user experience