You did not follow the guide.
CodeIgniter 4 is not just upload and boom.
First of all, you uploaded the project into a sub folder. Which can work but no, it’s a disaster. The guide asks you to upload the content of that folder, not the folder itself, into htdocs.
So the URL should definitely not have things like code4 or appstarter-4.6.4 in it.
Besides, you clearly haven’t done the .htaccess part, or done it correctly. Because if you did, you’ll not have the 403 error.
Why? Because what can be loaded is located inside the public folder, so you must use that piece of .htaccess to load from that folder. The project folder itself (code4) doesn’t have anything to load so it will always return a 403.
Finally, this
is not a 403 error. It’s a 500 error, which means that your code crashed. I hope you didn’t just move the index.php out of public because that is not how it works.