how to install CI4 in sub folder in shared hosting?

You should set the "DocumentRoot" option in HTTPD.conf your server settings, Example:
DocumentRoot = "%siteDir%/myapp/public/"
Or if you renamed the "public" folder to "myapp", then install something like this:
DocumentRoot = "%siteDir%/myapp/"
Where %siteDir% points to a domain folder.

I would change this world, but God doesn't give me the source.