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.

But you can serve more request on same hardware with other programming languages or php implementations such as php-pm, swoole or workerman.

even 15x - 50x times more requests.

pro novice