If you're using an American Wii U, 5.5.3U is the latest version. 5.5.4 is only available on the European Wii U.
To answer your question, 5.5.3U might not be required to play online on the Wii U. While the latest system version is needed for things like the eShop, online play is a little more lenient. I was playing online with 5.5.1U for quite awhile after 5.5.2U was released, for example.
That all being said, there's no reason not to update to the latest version.