Can CI 4 handle a thousand request?

It's 100% about the code you write and not Codeigniter. If you write bad code that is slow, it doesn't matter what framework you're using, it'll be slow. If you write good clean code, you can be on laravel, symfony, codeigniter or anything and serve hundreds of thousands or even millions of requests depending on your servers.

I disagree. Its all about the infrastructure. Supporting hundreds, thousands, millions of users has little to do with "clean code". Supporting a few hundred users is very different from supporting millions.

Slow database requests is almost always the reason a site is slow. There’s a lot to take into consideration when dealing with a lot of traffic.