Well, from my experience in Japanese language school in Japan, the reading material was quite often sad or dark there too. Although confirmation bias is probably at hand, haha. Iโve read all Ask volumes except the second and third one for level 4, and I donโt think there is a volume without a sad story, but certainly volumes without any that you could call happy.
Edit: Also yeah, many are fairytales or folk tales and they have that similar dark tone that European ones have too. ^^
Btw, for those who might be wondering if level 4 is a necessary/useful level of graded readers (when it comes to Ask), level 4 can easily be skipped because it is similarly complex to native material, in fact a lot of it is straight native material. The only benefit is that you know you wonโt find any especially hard grammar and that the vocabulary will be fairly standard (aka if youโve picked up a good base of vocabulary, it will only be a word here or there that are unknown to you). However, only really pick it up if youโve enjoyed earlier volumes for their content.
If anyone is interested in what Iโve thought of the Ask reader volumes including from level 1-3, check out the Graded Readers and Parallell texts (casual) book club. There is no central area to find my reviews, but searching for my username and Ask should probably bring them up. Also you can find how other people found those graded readers and a few others.