Class Name is variable does NOT work

This code work on CI3 but not on CI4

PHP Code:

class Foo
{
    public static $var2 = 1;
}

$classname = 'Foo';
echo $classname::$var2; // Outputs 1 


In CI4, it will return an error: Class 'Foo' not found
Could anyone please  tell me how to fix it. 

Thank you in advance.

Please note that if I change echo $classname::$var2; to echo Foo::$var2;, it will work/