My 4 year old loves to read, recently started taking karate, and also started building lego sets so this series of books was a natural fit. They are a little longer than the books we usually read but my son has a good attention span for stories & we just read 1 instead of the usual 2-3 at quiet time & bedtime. The stories do have some content that that could be too much for some younger kids (battle violence, skeleton bad guys) but my son is already a fan of power rangers & transformers (thanks daddy) and isnt fearful so were ok with the content. The storylines are interesting enough that I dont mind reading them over & over, unlike some of his books. He also received a couple ninjago lego sets for his bday & was excited to build them & act out things from the stories which is great for working on his fine motor. My only complaint is that the #2 reader leaves off with the ninjagos recovering the 1st of 4 weapons & this reader picks up with them having recovered 3 of the weapons, so my son keeps bugging me to get the "missing" book.