I have installed MySQL 8.0.4, but my Navicat for MySQL can not connect it, the warning is:
1251 - Client does not support authentication protocol requested by server; consider upgrading MySQL client.
How do I fix it?
From your warning message it looks like that your Navicat do not support MySQL 8.0.4.
Option 1
Just to be clear, the version of MySQL that you updated to 8.0.4 was released at 2018-01-23 and latest version of Navicat was released 2017-05-31 so 99,99% sure that it wont support latest MySQL, so either downgrade to 5.7.x version or just wait next release of Navicat.
Option 2 Check if there is a client for MySQL which is alrady supporting version 8.0.4. Hint here - I'm using SequelPro and it is not supporting it also. Hint #2 MySQL Workbench releasing quite often, meaning the soonish support of latest mysql is quite possible for tham
Option 3 You always have command line client, which is in some cases the best one, especially for learning reasons.
Regards