You can start off with
Little Witch Academia,
which has an official subtitled stream on Youtube. It's short, exciting, simple, and very safe.
If you want to go for something longer, try
Ashita no Nadja and
Heartcatch Precure. The latter has a lot of action and fighting, but no gore or blood splatter.
Looking at other people's recommendations, I actually advise
against Hyouka,
Clannad, and
Kanon. These series may appear safe and appropriate, but they deal with themes that I feel your 4-year-old brother would have trouble connecting to. Examples include:
- The jealously felt from untalented people who have a passion in a hobby towards people who have natural talent in that same hobby but a lack of interest.
- A character has their own job advancement cancelled just because their father was involved in a crime. (I had trouble understanding this even as a teenager!)
- A character has a terminal disease, and they are shown unsheathing a box cutter. The implication is that they plan to kill themself (this doesn't carry through), but a young child will not get this or empathize with the character.
In my opinion, there's something more important to "kid-friendly" entertainment than simply content such as violence and nudity: the subject matter should not be above their heads.
Ashita no Nadja and
Heartcatch Precure are targeted towards children in Japan, so they are fairly simple, although they don't patronize their audience like a lot of North American cartoons do. And
Little Witch Academia is pretty much for anyone.