Change data in Form Validation

In CI3 this works fine

// Function change first letter to capital
public function setUpperLetter ($str): string {
return mb_convert_case($str, MB_CASE_TITLE, 'UTF-8');
}

// Rule
'nick' => [
'label' => ' ',
'rules' => 'required|trim|max_length[50]|setUpperLetter'
],


In CI4 i got normal letter, is not possible do in form validation?