As visible on its official site, the 4th season has not yet been broadcasted on the original network of the series.

Hi, TV rights and licensing can change over time especially with a show distributed internationally.

It may never return to NPO in the Netherlands since they released Season 4 on TV 2 Charlie in the Netherlands. Do you plan to leave it blank forever and deprive your users of useful information?

See here: https://play.tv2.dk/serie/van-der-valk-tv2charlie/4

To change the origin network, we need a proof that NPO will not anymore broadcast this show.

At this point it would have to be by inference. It seems unlikely that it will be released on NPO given the time that has passed and another network in the same country already has season 4.

Suggestion: Add a feature to track the network by country, which can then have country/network specific air dates.

This is said to be broadcasted on NPO starting May 30th with 1 episode every friday.

https://www.bnnvara.nl/varagids/artikelen/van-der-valk-vierde-seizoen-gaat-in-mei-van-start-bij-de-npo