Where to create a file for custom filter css code in phalcon 4?

The error message is pretty clear:

Fatal error: Declaration of Frontend\Helpers\CssYUICompressor::filter($contents) must be compatible with Phalcon\Assets\FilterInterface::filter(string $content): string in /home/devmass/web/dev.loc/public_html/apps/frontend/helpers/CssYUICompressor.php on line 12

Taking a look at the definition of FIlterInterface: https://github.com/phalcon/cphalcon/blob/master/phalcon/Assets/FilterInterface.zep

You can see the input parameter MUST be a string, so you should refactor your filter class to match that:

namespace Frontend\Helpers;

use Phalcon\Assets\FilterInterface;

/**
 * Filters CSS content using YUI
 *
 * @param string $contents
 * @return string
 */
class CssYUICompressor implements FilterInterface
{
    protected $options;

    /**
     * CssYUICompressor constructor
     *
     * @param array $options
     */
    public function __construct(array $options)
    {
        $this->options = $options;
    }

    /**
     * @param string $contents
     *
     * @return string
     */
    public function filter(string $contents): string
    {
        // Write the string contents into a temporal file
        file_put_contents('temp/my-temp-1.css', $contents);

        system(
            $this->options['java-bin'] .
            ' -jar ' .
            $this->options['yui'] .
            ' --type css ' .
            'temp/my-temp-file-1.css ' .
            $this->options['extra-options'] .
            ' -o temp/my-temp-file-2.css'
        );

        // Return the contents of file
        return file_get_contents('temp/my-temp-file-2.css');
    }
}