> When it was implemented 4 byte unicode did not exist.
Incorrect. When UTF-8 was invented, it was actually variable up to 6 bytes in length, being capable of representing code points up to U+7FFFFFFF. It was only shortened to 4 bytes in 2003. There is no point in history where UTF-8 was only limited to 3 bytes.
Now that it does they can not change the names anymore.
It's the same as why windows is stuck with utf-16, because when they implemented it unicode was 2 bytes.