I enjoyed the entire article about exposing your children to cartoons in three different languages, but I was especially fascinated with the section about “filling gaps”. I think it is quite wonderful that your 4-year-old can perceive the gaps in his vocabulary in each of the languages, and that he purposely closes those gaps by watching the same cartoon in all three languages. I think that this is an excellent strategy.
To a much lesser extent, I sometime try a similar strategy in other media. If I find a magazine where all the articles appear twice, in two different languages, then I often try to read some of the articles in both languages. When I watch a foreign language film with English subtitles, then I listen carefully to the dialog to match it up with the printed text. Because I see the text in English a moment before I hear the spoken words, I can check for surprises in the words and phrases that I hear.