I guess what we are trying to say is upgrading a Mk4 would be fraught with potential compatibility problems.
It may be possible but would probably take quite a lot of expert knowledge to overcome all snags. Even a professional audio installer may lack the knowledge to integrate it into all of Hondas computer systems, and I doubt a Honda dealer would have the necessary experience either - or would charge an arm and a leg
Once an expert has succeeded others can copy. I think you will be lucky to find someone who has done it yet on a mk4 in Europe. I wouldnt be brave enough to be the first

You may have more luck with information from the Japanese market . In Japan the Jazz/Fit is popular with young drivers and sometimes sold with basic radio, with the expectation the buyers will upgrade it. But there may still be major specification differences to overcome.
Maybe a higher spec head unit and the necessary other bits removed from a scrapped car might be viable.