Avengers 4 will not feature the TV characters of the Marvel Cinematic Universe but the Inhumans characters were likely an a key original component of the battle against Thanos.
Originally dubbed Avengers: Infinity War Part II, the final film announced by Marvel Studios back when the curtain was pulled back on their Phase Three plans was following an Inhumans movie. Soon after, the studio gained the rights to Spider-Man, announced Spider-Man: Homecoming and Ant-Man and The Wasp, shifted the schedule to move Captain Marvel and Black Panther's release dates, and completely removed the Inhumans movie from the slate.
Each film to come before Avengers 4 and its Avengers: Infinity War predecessor seems to have been chosen to serve a purpose as the culmination of the Marvel Cinematic Universe comes to fruition. With only Ant-Man being an exception, each Marvel Studios film which released prior to Avengers: Infinity War served a purpose of building towards the narrative of Thanos' arrival. Ant-Man and its Quantum-verse, however, ire likely to be a crucial factor in Avengers 4. The Phase One, Two, and Three teases of Infinity Stones, character introductions, and location establishments were key to bringing the ensemble film to life.
For example, Guardians of the Galaxy thoroughly introduced Thanos and teased his search for the Infinity Stones. Avengers: Age of Ultron introduced the emotional narrative of the Vision being comprised of a Stone which Thanos would hunt. Black Panther introduced Wakanda, where the third act of Avengers: Infinity War took place.
Perhaps the final battle of Avengers: Infinity War was originally planned for the Inhuman home of Attilan, rather than Wakanda? Maybe the royal family being effected by Thanos' finger snap could have prompted their joining the effort to undo his evil deeds?
Charging toward Avengers 4, Captain Marvel has now taken the place of Inhumans as the last movie being released before the ensemble film. The Brie Larson heroine will play a considerable role in Avengers 4 (which the writers and directors detailed here), possibly indicating that the Inhumans characters might have been planned to do the same, along with their home.
As the story goes, though, the Inhumans characters were instead used on a television series which was canceled after one season. Panned by fans and critics alike, the show was a disaster and the characters will likely remain off-screen for some time before being re-introduced the the Marvel Cinematic Universe, should the studio ever choose to give them another shot.
Black Panther is available now on blu-ray and DigitalHD. Avengers: Infinity War is now playing theaters.