My Wii is also 4.3U I would run SYScheck on your system and upload the file to your SD card then run ModMii and drag the file to the program and it will tell you if your IOS is up to date if not you will see files it needs and to download them to your SD card after that a readme on how to update...
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day