My son is in fourth grade but I am not finding any science or history for his age.

This guy does a great job of introducing science to youngsters.

https://www.youtube.com/user/itsokaytobesmart

My 5 year old enjoys these science vids:  https://www.youtube.com/channel/UC-3SbfTPJsL8fJAPKiVqBLg

 You may or may not have to filter some of the content but these are well done:

https://www.youtube.com/user/lifenoggin

And I wish there were videos similar to the below dealing with the whole of human history.

https://www.youtube.com/watch?v=dGiQaabX3_o