I found that the history and other topics are aimed at teens and adults rather than younger students.
That said, my child has enjoyed the Crash Course History and Biology videos since age 11.
It would be nice if the site expanded with age-appropriate material on all the topics!

The appreciation of art does not really depend on the age of the student, but the part of these courses that are more likely to work with grade four students are, I think, The language can be too complicated, the progression too quick - and some of the Art history images could be disturbing to young people.

Music

Doodle music

Instruments of the orchestra (pick a few of the more common ones)

(Music basic, note values and such progresses much too rapidly for 4th graders).

Visual art

Possibly  Why look at art, What maps tell us, Is there a difference between art and craft, How can art make you analyze, 

and quite a lot of the videos, although, you might consider not using the sound and subtitles because the language can probably be a bit complicated, and sometimes too fast.

I haven't tried to make a curriculum for 4th graders based on the Khan Academy material, I've just picked a few examples I could have used with my own children when they were that age. 

World history has an intended audience of almost adults, and while I'm sure there are videos that could be used (again, you might want to turn off sound and substitute your own voice with a different text) it would mostly require a lot of work.

I hope this helps.