Accessing PHP library in CI4

Hello,

I am rewriting my old CI3 project to CI4.

I have a part where the app accesses the external PHP library and uses its functions.

it looks like this:

  

PHP Code:

        require_once('libwebtopay/WebToPay.php');

        try {
            $request = WebToPay::redirectToPayment(array( 


But in CI4, I get an error. 

Class 'App\Controllers\WebToPay' not found

It looks like CI4 thinks that WebToPay is a controller.

Is it possible to make CI4 read external PHP classes?