Season 4 now has a time frame of Fall 2020. We don’t know much more at this time but we do get hints at the end of season 3 that it’s following the same direction as the manga went. We are told that this is the Final Season. We also know the person writing the manga (Hajime Isayama) have said the Manga is entering it’s final arc as well.
The story seem simple to start. The last of Humanity has built huge walls to protect themselves from the Titan threat that can be found outside of the walls. Titans are gigantic humanoid creatures who devour humans seemingly without reason. But as time went on we found out more about the Titans, what is past the walls and what all has happen both within and outside of the great walls that protect the people.
We have done a review on the first three seasons that you can find Here